rng: initialize file descriptor to -1

The file descriptor is never initialized to -1, which makes rng-random
close stdin if an object is created and immediately destroyed.  If we
change it to -1, we also need to protect qemu_set_fd_handler from
receiving a bogus file descriptor.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Tested-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
