CA-92553: Thoroughly clear PCI attachments for stopped VMs

This should not be necessary if many-to-many relations in the DB
work properly. People have reported issues that may indicate that this is
not the case, but we have not yet found the root cause. Therefore, this
code is added "just to be sure".

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Merge pull request #1073 from robhoes/gpu

CA-92553: Thoroughly clear PCI attachments for stopped VMs