MAINTAINERS: extend coverage for "THE REST"

MAINTAINERS: extend coverage for "THE REST"

As agreed upon in offlist discussion among the committers, make all
committers eligible to approve changes to code not having its
maintainership covered explicitly. (For committers to make changes to
such code, generally an ack from a second committer is going to be
required.)

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>