Clarify that:

- for protocols the brdv_file_open function is used instead
of bdrv_open;

- when protocol_name is set, a driver should expect
to be given only a filename and no other options.
Signed-off-by: NFabiano Rosas <farosas@linux.vnet.ibm.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>