qxl: don't create too large primary surface

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>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>