libxc: convert gnttab interfaces to use an opaque handle type

libxc: convert gnttab interfaces to use an opaque handle type

The xc_interface previously passed to xc_gnttab_* was only used for
logging which can now be done via the xc_gnttab handle instead.

This makes the interface consistent with the changes made to the main
interface in 21483:779c0ef9682c.

Also update QEMU_TAG to pull in the corresponding qemu change.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>