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