qmp: add test tool for QMP

Anthony wrote this quickly to aid in testing.  It's similar to qmp-shell with
a few important differences:

1) It is not interactive.  That makes it useful for scripting.

2) qmp-shell:

(QEMU) set_password protocol=vnc password=foo

3) qmp:

$ qmp set_password --protocol=vnc --password=foo

4) Extensible, git-style interface.  If an invalid command name is
passed, it will try to exec qmp-$1.

5) It attempts to pretty print the JSON responses in a shell friendly
format such that tools can work with the output.

Hope others will also find it useful.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
