kvm_init didn't set return value after create vm failed

And kvm_ioctl(s, KVM_CREATE_VM, 0)'s return value can be < -1,
so change the check of vmfd at label 'err'.

Signed-off-by: Xu He Jie <xuhj@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
