qxl/update_area_io: cleanup invalid parameters handling

This cleans up two additions of almost the same code in commits
511b13e2c9 and ccc2960d654.  While at it, make error paths
consistent (always use 'break' instead of 'return').

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Cc: Dunrong Huang <riegamaths@gmail.com>
Cc: Alon Levy <alevy@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
