Add support for pretty-printing response in qmp-shell

Add support for pretty-printing response in qmp-shell

Add a '-p' arg to the QMP/qmp-shell test program, which uses
the python pprint module to pretty-print the dictionary
returned from a command

  $ qmp-shell -p /tmp/qemu
  Welcome to the QMP low-level shell!
  Connected to QEMU 1.1.50

  (QEMU) query-cpus
  {   u'return': [   {   u'CPU': 0,
                         u'current': True,
                         u'halted': True,
                         u'pc': 1048556,
                         u'thread_id': 7108}]}

Signed-off-by: Daniel P. Berrange <[email protected]>
Signed-off-by: Luiz Capitulino <[email protected]>