Commit aa2cc7 modified a previously unnecessary but innocuous check
for interface IP address during interface update incorrectly, causing
all attempted updates (e.g. changing link state) to interfaces of
type='ethernet' for QEMU to fail.

This patch fixes the issue by completely removing the check for IP
address, which is pointless since QEMU doesn't support setting
interface IP addresses from the domain interface XML anyway.
Signed-off-by: NVasiliy Tolstov <v.tolstov@selfip.ru>
Signed-off-by: NLaine Stump <laine@laine.org>