When all SAN targets are completely unreachable, there will be a
natural delay between reopening attempts due to the network connection
timeout on the unreachable targets.

However, some SAN targets may accept connections instantly and report
a temporary unavailability by e.g. failing the TEST UNIT READY
command.  If all targets are behaving this way then there will be no
natural delay, and we will attempt to saturate the network with
connection attempts.

Fix by introducing a small delay between attempts.

Signed-off-by: Michael Brown <mcb30@ipxe.org>