drivers: xen: gnttab: process gnttabs in reverse order

The Xen extends domain grant tables every time domain requests gnttab
basing on gnttab idx. If idx > xen_current_max_gnttab_idx the Xen extends
grant table so that idx <= xen_current_max_gnttab_idx. The growing grant
tables on every hypercall is a bit costly operation and it also results in
the bunch of log messages:

(XEN) xen-source/xen/common/grant_table.c:1909:d0v0 Expanding d0 \
  grant table from 1 to 2 frames

This patch changes gnttab processing from gnttab max_idx to low_idx, so the
first hypercall has the largest index, ensuring that the grant table will
grow only once. It also reduces number of log messages.

Signed-off-by: Grygorii Strashko <grygorii_strashko@epam.com>
Signed-off-by: Dmytro Firsov <dmytro_firsov@epam.com>