monitor: use qmp_dispatch()

Replace the old manual dispatch and validation code by the generic one
provided by qapi common code.

Note that it is now possible to call the following commands that used to
be disabled by compile-time conditionals:
- dump-skeys
- query-spice
- rtc-reset-reinjection
- query-gic-capabilities

Their fallback functions return an appropriate "feature disabled" error.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20160912091913.15831-16-marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
