qemu-io: add flag to mark files growable

Add a -g flag to the open command and the main qemu-io command line to
allow opening a file growable.  This is only allowed for protocols,
mirroring the limitation exposed through bdrv_file_open.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
