XenAPI: Tolerate multiple coalesces

VHD coalescing might coalesce more than one VDI while waiting
and might coalesce the 'grandparent' before the parent.  Wait
for the parent to be any of the original tree instead of just
the original parent.

Closes bug: 1317792

Change-Id: I82c4db0e8a36c41a86adf4bd32304a4bfdabebbf
Merge "XenAPI: Tolerate multiple coalesces"