qemu-io: use BDRV_O_FILE to implement the growable open option

Instead of doing our own check for protocols which fails because raw isn't
formally a protocol but special cased in find_protocol specify the BDRV_O_FILE
option to use the same code as bdrv_file_open does.

While we're at it also add the missing documentation for -g to the main
qemu-io help string.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>