drop "domain_" prefix from struct domain's dirty CPU mask

drop "domain_" prefix from struct domain's dirty CPU mask

It being a field of struct domain is sufficient to recognize its
purpose.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>