kvm: remove hard dependency on pci

The msi routing code in kvm calls some pci functions: provide
some stubs to enable builds without pci.

Also, to make this more obvious, guard them via a pci_available boolean
(which also can be reused in other places).

Fixes: e1d4fb2de ("kvm-irqchip: x86: add msi route notify fn")
Fixes: 767a554a0 ("kvm-all: Pass requester ID to MSI routing functions")
Reviewed-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
