Do not prevent coalesce if only one VDI is corrupted

Currently, if only one VDI is corrupted on a SR (or without parent or any other error reported by `vhd-util`), the coalesce process isn't started.

This is not a good behaviour, because it's likely all other VDIs can be coalesced.

For what I understand, you are using a kind of vhd-util check/scan on *.vhd (for a file based SR for example), and if there is an error, coalesce won't be triggered.

Ideally, you should be able to make the check/scan on each file, and coalesce those are working and leave the "broken" ones.