QMP: Allow dot separated dict path arguments in qmp-shell

QMP: Allow dot separated dict path arguments in qmp-shell

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: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>