kvm: Make _GetKVMVersion() return integers

The current code returns the major, minor and rev portions of the version of
KVM as strings. Returning them as integers makes it easier to compare.

Signed-off-by: Miguel Di Ciurcio Filho <miguel.filho@gmail.com>
Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>