qxl: don't create too large primary surface

qxl: don't create too large primary surface

commit c572aaf46f71f63ae5914d4e194a955e0ba1b519 upstream.

Limit primary to qemu vgamem size, to avoid reaching
qemu guest bug "requested primary larger than framebuffer"
on resizing screen too large to fit.

Remove unneeded and misleading variables.

Related to:
https://bugzilla.redhat.com/show_bug.cgi?id=1127552

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>