networkd: report IPv4 address changes to xapi (as well as IPv6)

DHCP should now work well on most distros, and we can now get rid of the patch
to dhclient-script in dom0.hg (which only applies to a certain CentOS version).

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Merge pull request #1340 from robhoes/cp5153

networkd: report IPv4 address changes to xapi (as well as IPv6)