Add check for more than one RTA_OIF, even though this is rather
unlikely.

Get rid of the buggy switch / break as this code won't need to
handle more attributes.

Use VIR_WARNINGS_NO_CAST_ALIGN to fix impossible to fix
util/virnetdevip.c:560:17: error: cast increases required alignment of target type [-Werror=cast-align]

(cherry picked from commit b202c39a)