iotests: Move qmp_to_opts() to VM

qmp_to_opts() used to be a method of QMPTestCase, but recently we
started to add more Python test cases that don't make use of
QMPTestCase. In order to make the method usable there, move it to VM.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>