qemu fails when the same `fw_cfg` options is used more than once.
Since the current logic always adds a new option on each machine load
this will fail on the second start.
We can fix this by checking if the option is already set and replace but
I think it is easier to just not commit the option in the config and add
it dynamically on start. User that hit this bug have to recreate the
machine.
[NO NEW TESTS NEEDED]
Fixes#14636Fixes#14837
Signed-off-by: Paul Holzinger <pholzing@redhat.com>
3ce0709f37