libxl: set the default grant/maptrack frames at structure init

libxl: set the default grant/maptrack frames at structure init

libxl_domain_build_info had both the maptrack and grant frames set to
0 by default, forcing the client of libxl to set a sane default.

This is not backwards compatible, so instead initialize both
max_grant_frames and max_maptrack_frames to a sane default (ie: like
previous behavior).

This fixes the libvirt tests in osstest.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>