xl: add global grant limit config items

xl: add global grant limit config items

Add xl.conf config items for default values of grant limits:

max_grant_frames will set the default for the maximum number of grant
frames for a domain which will take effect if the domain's config file
doesn't specify a value. If max_grant_frames isn't set in xl.conf it
will default to 32 for hosts with all memory below 16TB and to 64 for
hosts with memory above 16TB.

max_maptrack_frames will set the default for the maximum number of
maptrack frames for a domain. If max_maptrack_frames isn't set in
xl.conf it will default to 0, as normally only backend domains need
maptrack frames.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>