As another convenience to allow using commands that expect a dict as
argument, this patch adds support for foo.bar=value syntax, similar to
command line argument style:

  (QEMU) blockdev-add options.driver=file options.id=drive1 options.filename=...
Signed-off-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>