vm: Fix mistyped 'portMirroring'

This is related to 8225ae9aeebe88a0f891b390e3c0177671515210
(vmDevices: add __slots__ to devices) and was hiding
the bug at https://bugzilla.redhat.com/1011887

Change-Id: I5d7fb6dd4aaeda37aefebc57bcf0d5007bb12290
Signed-off-by: Ondřej Svoboda <osvoboda@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/25075
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>