qmp: expose list of supported character device backends

Introduce 'query-chardev-backends' QMP command which lists all
supported character device backends.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
