engine: Allow vlan to serve as a migration network

In order for a nic device to be entitled to serve as the
underlying interface for a migration network, it should
be UP. The nics statuses are reported by the getVdsStats
verb of VDSM which doesn't report it for vlan devices.
Therefore, until the statistics will be collected for vlans,
we'll allow migration to go over the vlan if its underlying
nic is UP.

Change-Id: Icdfcca442d40d8f2c3b6b28dc9f520e286b7e8f2
Bug-Url: https://bugzilla.redhat.com/1014013
Signed-off-by: Moti Asayag <masayag@redhat.com>