scsi-hd: fix property unset case

Commit c53659f0 ("BlockConf: Call backend functions to detect geometry
and blocksizes") causes a segmentation fault on the invalid
configuration of a scsi device without a drive.

Let's check for conf.blk before calling blkconf_blocksizes. The error
will be handled later on in scsi_realize anyway.

Reported-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Ekaterina Tumanova <tumanova@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
