engine: Adjust MigrateVmCommand to accept any vlan device name

Changes done in the command-
1. The command uses iface.getBaseInterface() and iface.getVlanId() instead
of determining this values from the vlan device name (since its format is
no longer- baseIface.vlanId).
2. The command uses the new methods indroduced in NetworkUtils instead of
the old ones (that should be removed in a following patch).

Change-Id: I0d8989320d39ef74d55574cfb1da0f5efe4ac6ec
Bug-Url: https://bugzilla.redhat.com/1091863
Bug-Url: https://bugzilla.redhat.com/999975
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>