tools/libxc, xen/x86: Added xc_set_mem_access_multi()

tools/libxc, xen/x86: Added xc_set_mem_access_multi()

Currently it is only possible to set mem_access restrictions only for
a contiguous range of GFNs (or, as a particular case, for a single GFN).
This patch introduces a new libxc function taking an array of GFNs.
The alternative would be to set each page in turn, using a userspace-HV
roundtrip for each call, and triggering a TLB flush per page set.

Signed-off-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tamas K Lengyel <tamas@tklengyel.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>