virtio-scsi: dataplane: fail setup gracefully

The dataplane code is currently doing a hard exit on various setup
failures. In practice, this may mean that a guest suddenly dies after
a dataplane device failed to come up (e.g., when a file descriptor
limit is hit for the nth device).

Let's just try to unwind the setup instead and return.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
