libqtest: escape strings in QMP commands, fix leak

libqtest is using g_strdup_printf to format QMP commands, but
this does not work if the argument strings need to be escaped.
Instead, use the fancy %-formatting functionality of QObject.
The only change required in tests is that strings have to be
formatted as %s, not '%s' or \"%s\".  Luckily this usage of
parameterized QMP commands is not that frequent.

The leak is in socket_sendf.  Since we are extracting the send
loop to a new function, fix it now.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
