xen, fbfront: add support for specifying size via xenstore

Today xen-fbfront supports specifying the display size via module
parameters only. Add support for specifying the size via Xenstore in
order to enable doing this easily via the domain's Xen configuration.

Add an error message in case the configured display size conflicts
with video memory size.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>