Let VMAC interface flags (up/down status) follow base interface.

When using a VMAC interface, this fix will reflect the base interface flags,
i.e. up/down status, to the VMAC interface. This is useful when using sync
groups (in combination with VMAC) and a link for one of the members in the
MASTER sync group goes down. Before this fix, this member will not detect
the link fault, due to that the VMAC interface always is UP regardless of
the actual status of the base interface, and the sync group will continue to
be MASTER as if nothing has happend. This fix will however reflect the status
of the base interface onto the VMAC interface, so if the link goes down the
member will transit to FAULT state, which will make the sync group transit
to BACKUP state.

Signed-off-by: Jonas Johansson <jonasj76@gmail.com>
Merge pull request #37 from jonasj76/vmac-reflect-flags

Let VMAC interface flags (up/down status) follow base interface.