qemu-io: Extended "--cmd" description in usage text

qemu-io: Extended "--cmd" description in usage text

It's not clear from the usage description that "--cmd" option accepts
its argument as a string, so any special symbols have to be quoted from
the shell.

Updates in usage text:
 - Specified parameter format for "--cmd" option.
 - Added an instruction how to get help for "--cmd" option.

Signed-off-by: Maria Kustova <maria.k@catit.be>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>