qxl: allow to specify head limit to qxl driver

This patch allow to limit number of heads using qxl driver. By default
qxl driver is not limited on any kind on head use so can decide to use
as much heads.

libvirt has this as a video card parameter (actually set to 1 but not
used). This parameter will allow to limit setting a use can do (which
could be confusing).

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>