libxl: Wait for ballooning if free memory is increasing

libxl: Wait for ballooning if free memory is increasing

During domain startup, all required memory ballooning must complete
within a maximum window of 33 seconds (3 retries, 11 seconds of delay).
If not, domain creation is aborted with a 'failed to free memory' error.

In order to accommodate large domains or slower hardware (which require
substantially longer to balloon memory) the free memory process should
continue retrying if the amount of free memory is increasing on each
iteration of the loop.

Signed-off-by: Mike Latimer <mlatimer@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>