Fix types used in xc_memshr_* functions

Fix types used in xc_memshr_* functions

Currently the memshr functions use uint32_t for the domid.  This actually leads
to a funny domid_t -> uint32_t -> domid_t sequence of casts.  This patch
updates the API functions to be domid_t in the first place.

No tools need to be modified with this patch, the casts were implicit.

Signed-off-by: Adin Scannell <adin@scannell.ca>
Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Acked-by: Ian Jackson <ian.jackson.citrix.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>