xen: Report grant table v1/v2 capabilities to the toolstack

xen: Report grant table v1/v2 capabilities to the toolstack

In order to let the toolstack be able to set the gnttab version on a
per-domain basis, it needs to know which ABIs Xen supports.  Introduce
XEN_SYSCTL_PHYSCAP_gnttab_v{1,2} for the purpose, and plumb in down into
userspace.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>
Reviewed-by: Ian Jackson <iwj@xenproject.org>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Releae-Acked-by: Ian Jackson <iwj@xenproject.org>