qxl: stop dirty loging when not in vga mode

Tested with linux guest. Not sure how to check actual performance affect
of this. Checked with the previously send traceevent that the kvm ioctl
to start/stop dirty logging is being called.
(KVM_SET_USER_MEMORY_REGION).

Signed-off-by: Alon Levy <alevy@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
