introduce gnttab_max_frames and gnttab_max_maptrack_frames command line options

introduce gnttab_max_frames and gnttab_max_maptrack_frames command line options

Introduce gnttab_max_maptrack_frames: a new Xen command line option to
specify the max number of maptrack frames per domain.
Deprecate the old gnttab_max_nr_frames and introduce gnttab_max_frames
instead, that doesn't affect the maptrack. Keep gnttab_max_nr_frames for
compatibility.

Rename internally max_nr_grant_frames to max_grant_frames to avoid
confusions.

Introduce DEFAULT_MAX_MAPTRACK_FRAMES, that is completely independent
from max_nr_grant_frames.

Remove MAX_MAPTRACK_TO_GRANTS_RATIO that is now only used in one place
for compatibility.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>