s390/sclpconsole: prevent char layer callback during initialization

Starting a qemu with an sclp console and pressing a key very early
can result in
"qemu-system-s390x: hw/s390x/sclpconsole.c:60: receive_from_chr_layer:
Assertion `scon->iov' failed."

Lets make sure that the init process is finished, since the iov is
allocated after CHR_EVENT_OPENED by also checking for scon->iov.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
