qapi: Split dump.json off misc.json

Move commands dump-guest-memory, query-dump,
query-dump-guest-memory-capability with their types from misc.json to
new dump.json.  Add dump.json to MAINTAINERS section "Dump".

Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190619201050.19040-15-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>