vm: drop redundant hasattr()

I don't see how it is possible for a Vm instance to *not* have the
updateGuestCpuRunning method, therefore I remove the hasattr() check
before to call it.

Change-Id: I23743b7c6f4a8b71a301d02670c348d53f23f124
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29112
Reviewed-by: Dan Kenigsberg <danken@redhat.com>