qxl: add a connector property to denote hotplug should rescan modes.

So GNOME userspace has an issue with when it rescans for modes on hotplug
events, if the monitor has no EDID it assumes that nothing has changed on
EDID as with real hw we'd never have new modes without a new EDID, and they
kind off rely on the behaviour now, however with virtual GPUs we would
like to rescan the modes and get a new preferred mode on hotplug events
to handle dynamic guest resizing (where you resize the host window and the
guest resizes with it).

This is a simple property we can make userspace watch for to trigger new
behaviour based on it, and can be used to replaced EDID hacks in virtual
drivers.

Acked-by: Marc-André Lureau <marcandre.lureau@gmail.com> (on irc)
Signed-off-by: Dave Airlie <airlied@redhat.com>