engine: Rollback Mac Allocation in AddVmInterface

This patch makes sure that the Mac address is
released in case user requested to add and activate
nic, but activation of nic failed.

Change-Id: Ic982eba73663b8ee47524ea3814f7255ec7a1a1c
Bug-Url: https://bugzilla.redhat.com/889797
Signed-off-by: Muli Salem <msalem@redhat.com>