qemu: More qemu_monitor_json cleanups

qemu:  More qemu_monitor_json cleanups

Recent adjustments to the code produced a litany of coverity false
positives, but only because the "standard" procedure of setting a
variable to NULL after it was assigned to something else and keeping
the *Free/*FREE call in the cleanup path wasn't kept. So this patch
makes those adjustments (assign variable to NULL and remove the if
'ret < 0' condition to clean it up).

Signed-off-by: John Ferlan <jferlan@redhat.com>