QMP: allow JSON dict arguments in qmp-shell

QMP: allow JSON dict arguments in qmp-shell

qmp-shell hides the QMP wire protocol JSON encoding from the user.  Most
of the time this is helpful and makes the command-line human-friendly.

Some QMP commands take a dict as an argument.  In order to express this
we need to revert back to JSON notation.

This patch allows JSON dict arguments in qmp-shell so commands like
blockdev-add and nbd-server-start can be invoked:

  (QEMU) blockdev-add options={"driver":"file","id":"drive1",...}

Note that spaces are not allowed since str.split() is used to break up
the command-line arguments first.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>