networkd: do signal xapi when an IP address is removed

BobB pointed out that when a DHCP lease expires, the IP address may go away.
Therefore, a signal to xapi for it to update its DB makes sense.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Merge pull request #1343 from robhoes/ipmon

networkd: do signal xapi when an IP address is removed