qxl: fix pre-save logic

qxl: fix pre-save logic

Oops.  Logic is backwards.

Fixes: 39b8a183e2f3 ("qxl: remove assert in qxl_pre_save.")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/610
Resolves: https://bugzilla.redhat.com//show_bug.cgi?id=2002907
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210910094203.3582378-1-kraxel@redhat.com>
(cherry picked from commit eb94846280df3f1e2a91b6179fc05f9890b7e384)
Signed-off-by: Michael Roth <michael.roth@amd.com>