net/announce: Allow optional list of interfaces

Allow the caller to restrict the set of interfaces that announces are
sent on.  The default is still to send on all interfaces.

e.g.

  { "execute": "announce-self", "arguments": { "initial": 50, "max": 550, "rounds": 5, "step": 50, "interfaces": ["vn2", "vn1"] } }

This doesn't affect the behaviour of migraiton announcments.

Note: There's still only one timer for the qmp command, so that
performing an 'announce-self' on one list of interfaces followed
by another 'announce-self' on another list will stop the announces
on the existing set.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>