block: Expose host_* drivers in blockdev-add

All the functionality to use the host_device, host_cdrom and host_floppy
drivers is already there, they just need to be added to the schema.

The block driver names containing underscores are preexisting and cannot
be changed without breaking command line compatibility.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
