bpcdump: allow removal of any VE backup

When looking to free space for the new backup, allow removal of older
backups for any VE, not just for the current VEID.  Since the typical
use case now is to back up just a single BackupPC container, a change in
VEID is going to be due to a replacement BackupPC container, such as in
a major upgrade scenario.

If these scripts are in fact used in the future to back up multiple
BackupPC containers, additional work will be required on the script.