QOM backends can refer to chardevs, but not vice versa.  So
process -chardev and -fsdev options before -object

This fixes the rng-egd backend to virtio-rng.

Reported-by: Amos Kong <akong@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>