XenAPI: Monitor the GC when coalescing

Add a new host plugin to monitor whether the garbage collector is running.
This is then used by the wait_for_vhd_coalesce function to know whether
waiting any more is likely to produce the expected results.

Increased the number of times we loop waiting for coalesce as the loop
now has knowledge that something is still happening.

DocImpact
Change to parameter default.

Change-Id: Idcc0738945bd1aee8cdb52c9cfabd798f85c31db
Closes-bug: 1258169
Merge "XenAPI: Monitor the GC when coalescing"