[xenapi] Cooperatively yield during sparse copy

On a resize down the sparse copy blocks other operations from running on
the compute node, and can take a long time to finish.  This patch adds a
greenthread yield to not block other things from happening on the
compute node.

Change-Id: I1287a6a0bb3157ad0e5eafe53b02db21849650fa
Merge "[xenapi] Cooperatively yield during sparse copy"