qemu-io: Factor out qemuio_command

It's duplicated code. Move it to qemu-io-cmds.c because it's not
dependent on any static data of the qemu-io tool.

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