Never overwrite a QemuOpt

Never overwrite a QemuOpt

Rather than overwriting a QemuOpt, just add a new one to the tail and
always do a reverse search for parameters to preserve the same
behaviour. We use this order so that foreach() iterates over the opts
in their original order.

This will allow us handle options where multiple values for the same
parameter is allowed - e.g. -net user,hostfwd=

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>