core: increment the vm generation when disk is being unplugged

When a disk is being detached from a vm the vm generation should be
incremented otherwise the vm's ovf won't be updated.

Change-Id: I91bb0b562907e8d5be4daa8bed97acbf148fae34
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1057108
Signed-off-by: Liron Aravot <laravot@redhat.com>