xen: lock target domain in do_domctl common code

xen: lock target domain in do_domctl common code

Because almost all domctls need to lock the target domain, do this by
default instead of repeating it in each domctl. This is not currently
extended to the arch-specific domctls, but RCU locks are safe to take
recursively so this only causes duplicate but correct locking.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: Jan Beulich <jbeulich@suse.com>
Committed-by: Keir Fraser <keir@xen.org>