XenAPI: Wait for VDI on introduce

Some SRs, particularly iSCSI connections, are slow to see the VDIs.

Introduce a sleep when a VDI that we're definitely expecting is not
present on the SR.

Change-Id: I7c71f1c7b811603aeacbb34c913dc236bba2132a
Closes-bug: 1274088
Merge "XenAPI: Wait for VDI on introduce"