xen: add a lock for the mapcache

Extend the existing dummy mapcache_lock/unlock macros to cover all of
xen-mapcache.c.  This prepares for unlocked memory access, when parts
of exec.c will not be protected by the BQL.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>