MAINTAINERS: add entry for QEMU

Gabriel merged support for QEMU FW CFG interface, but there's apparently
no official maintainer. It's also possible that this will grow more
interfaces in future.  I'll happily co-maintain it and handle pull
requests together with the rest of the PV stuff I maintain.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Gabriel Somlo <somlo@cmu.edu>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Gabriel Somlo <somlo@cmu.edu>