engine: Release Mac from Pool

This patch makes sure the Mac address is
released from pool in case the Update/Add
VmInterfaceCommand fails.

It moves the allocation of Mac Address from the
canDoAction to the execute method, allowing
rollback if necessary.

Change-Id: Idcec5bbd129f4413ec077102379a417bfd945bc3
Signed-off-by: Muli Salem <msalem@redhat.com>