1. A computer implemented method for reducing risk of service interruptions for one or more virtual machines (VMs) in a computing environment, the method comprising: computing a placement scheme for placing at least one VM on one or more hosts according to a set of placement constraints defined for the VM, wherein the set of placement constraints comprises at least one availability constraint defined for the VM, wherein the availability constraint designates a N resiliency level, wherein N corresponds to number of host failures that may occur before the services provided by the VM are interrupted, evacuating the VM from a first host to a second host in a computing environment, in response to determining that the first host pertains to a group of up to N hosts susceptible to failure, wherein said VM is placed on first host in accordance to the placement constraints, wherein said VM can be placed on a second host also in accordance to the placement constraints, wherein N is the designated N resiliency level for said VM, further comprising pre-allocating shadow VMs for the VM to minimize risk of service interruption for said VM due to failure of one or more hosts hosting the VM wherein: h denotes a group of hosts; VM(x) denotes a VM x; P(h,x) denotes placement of x on h; such that P(h,x)=1 if x is placed on h and P(h,x)=O; Otherwise: R(x,d) denotes resource requirement of VM(x) for a resource type d; C(h,d) denotes the capacity on host h of resource type d; the method further comprising, for each VM, adding a set of F shadow VMs: {VM(x,1), VM(x,2). .. , VM(x,F)}; wherein F is the number of host failures from which the VM is protected; wherein: VM(x,O) denotes an original VM(x) A j-shadow is a shadow of the form VM(x,j) for any x P(h,x,j)=1 if shadow VM(x,j) is placed on h the method further comprising constructing evacuation mapping in case of host failures by: numbering each failed host from 1 to F; for j=1 to F performing the following: (a) for x on host hG) setting evacuation target of x to the host containing the shadow VM(x,j); (b) for shadow VM(x,i) on host hG) doing the following: 1. converting shadow VM(x,j) to VM(x,i); and 2. if i<j then updating the evacuation target of x; the method further comprising constructing additional constraint rules by: for any VM x and any i!=j, set VM(x,i) as anti-collocated with VM(x,j), wherein: i,j may be 0; if VM(x) and VM(y) are anti-collocated then set VM(x,i) as anti-collocated with VM(y,j) for any i!=j. If VM(x) is anti-located with host h, then also VM(x,i) is anti-located with host h, for any i; for any resource type d and any host h, sumx (P (h,x)*R(x,d))+sum j (max h′!=h {sum y,i (P(h,y,j)*P(h′,y)*R(y,d)+P(h,y,j)*P (h′,y,i)*R (y,d))}) is prohibited from being greater than C(h, d) (P310d) h′ refers to any host that is different from h; sum x refers to the amount of resources to be allocated to VM x on h; sum j refers to the amount of resources to be allocated to shadow VMs on h; max h′!=h refers to the maximum amount of resources to be allocated to any VM on any h′; and sum y,i refers to the amount of resources to be allocated to the shadow VMs on h′ subject to: setting rule 1 such that 2 shadows of the same VM are not placed on the same host; setting rule 2 such that anti-collocation and anti-location apply to shadows, wherein a case i=j for anti-collocation is excluded, such that anti-collocated VMs have a different failed host number during evacuation; setting rule 3 as the resource capacity constraints such that: a left sum is the resource used for the real VMs on h, a right sum is the resource used for the shadows, shadows with different failure numbers (the sum over j) do not overlap; for each failure number j, summing the worst case host failure (both the shadows VM(y,j) on h corresponding to VM(y) on h′, and the shadows VM(y,j) that have a ‘brother’ shadow VM(y,i) (for any i) on host h′ to guarantee space on h both for the VMs of h′ (when h′ will be assigned as j) and for VMs of other hosts that have a shadow on h′ and a j-shadow on h, and due the Convert (in case h′ fails) will also be routed to their j-shadow; wherein due to rule 1, P(h,y,j) is 1 for at most one j on host h, so that all sets of J hosts are not maximized, wherein two different failure numberings cause evacuation of disjoint sets of VMs to the host h.