qmp-commands: fix incorrect uses of ":O" specifier

As far as the QMP parser is concerned, neither the 'O' nor the 'q' format specifiers
put any constraint on the command.  However, there are two differences:

1) from a documentation point of view 'O' says that this command takes
a dictionary.  The dictionary will be converted to QemuOpts in the
handler to match the corresponding HMP command.

2) 'O' sets QMP_ACCEPT_UNKNOWNS, resulting in the command accepting invalid
extra arguments.  For example the following is accepted:

   { "execute": "send-key",
        "arguments": { "keys": [ { "type": "qcode", "data": "ctrl" },
                                 { "type": "qcode", "data": "alt" },
                                 { "type": "qcode", "data": "delete" } ], "foo": "bar" } }

Neither send-key nor migrate-set-capabilities take a QemuOpts-like
dictionary; they take an array of dictionaries.  And neither command
really wants to have extra unknown arguments.  Thus, the right
specifier to use in this case is 'q'; with this patch the above
command fails with

   {"error": {"class": "GenericError", "desc": "Invalid parameter 'foo'"}}

as intended.

Reported-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>