Add new exception to retryables in SolidFire driver

Add new exception to retryables in SolidFire driver

Newer versions of SolidFire Element OS introduce an
internal snapshot mechanism during cloning that also
includes a new exception (xNotReadyForIO).

This can be encountered when programmatically cloning
hundreds of large volumes. Typically this will succeed
on a retry of the clone command, so we should add this
new exception to our list of retryable exceptions in
the SolidFire driver.

Change-Id: I4e1ddf02087499b4efff6eabf2310a245bef93c9
Closes-Bug: #1451505