MAINTAINERS: Remove and change David Gilbert maintainer entries

MAINTAINERS: Remove and change David Gilbert maintainer entries

I'm leaving Red Hat next week, so clean up the maintainer entries.

'virtiofs' is just the device code now, so is pretty small, and
Stefan is still a maintainer there.

'migration' still has Juan.

For 'HMP' I'll swing that over to my personal email.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Acked-by: Dr. David Alan Gilbert <dave@treblig.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Zhang Chen <chen.zhang@intel.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 20230330095524.37691-1-dgilbert@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>