vdpa_sim_blk: use dev_dbg() to print errors

Use dev_dbg() instead of dev_err()/dev_warn() to avoid flooding the
host with prints, when the guest driver is misbehaving.
In this way, prints can be dynamically enabled when the vDPA block
simulator is used to validate a driver.

Suggested-by: Jason Wang <jasowang@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20220630153221.83371-2-sgarzare@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>