MAINTAINERS: Add Fam and Jsnow for Bitmap support

MAINTAINERS: Add Fam and Jsnow for Bitmap support

These files are currently unmaintained.

I'm proposing that Fam and I co-maintain them; under the model that
whomever between us isn't authoring a given series will be responsible
for reviewing it.

Signed-off-by: John Snow <jsnow@redhat.com>
Acked-by: Fam Zheng <famz@redhat.com>
Acked-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>