kvm: Add more VM stats

kvm: Add more VM stats

This changeset adds the following stats to KVM:
 * numVMHalfEntries: Number of entries into KVM to finalize pending
   IO operations without executing guest instructions. These typically
   happen as a result of a drain where the guest must finalize some
   operations before the guest state is consistent.
 * numExitSignal: Number of VM exits that have been triggered by a
   signal. These usually happen as a result of the timer that limits
   the time spent in KVM.