qmp: allow setting properties to empty string in qmp-shell

The qmp-shell property parser currently rejects attempts to
set string properties to the empty string eg

  (QEMU) migrate-set-parameters  tls-hostname=
  Error while parsing command line: Expected a key=value pair, got 'tls-hostname='
command format: <command-name>  [arg-name1=arg1] ... [arg-nameN=argN]

This is caused by checking the wrong condition after splitting
the parameter on '='. The "partition" method will return "" for
the separator field, if the seperator was not present, so that
is the correct thing to check for malformed syntax.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <20170302122429.7737-1-berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
