Some SCSI targets (observed with an EMC CLARiiON Fibre Channel target)
will not respond to commands correctly until a TEST UNIT READY has
been issued.  In particular, a READ CAPACITY (10) command will return
with a success status, but no capacity data.

Fix by issuing a TEST UNIT READY command automatically, and delaying
further SCSI commands until the TEST UNIT READY has succeeded.

Reported-by: Hadar Hen Zion <hadarh@mellanox.co.il>
Signed-off-by: Michael Brown <mcb30@ipxe.org>