gdbsx_guestmemio: allow it to check domain

gdbsx_guestmemio: allow it to check domain

gdbsx_guest_mem_io() does not get d passed, it expects to handle the
domain lookup itself. Specifically, the caller of
XEN_DOMCTL_gdbsx_guestmemio is expected to use DOMID_IDLE to interact
with the hypervisor, rather than a domain, which doesn't interact well
with with the domain rcu lock.

Signed-off-by: Don Slutz <dslutz@verizon.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>