CA-102329: VDI.delete: avoid duplicate GC restarts

In VDI.delete, there is a race condition where two threads attempt to
re-start a running GC/coalesce daemon, because re-starting the
GC/coalesce is done by blindly writing a value to a file (see IPC.py).
If one thread finds that there is re-start pending, there's no need to
issue another GC/coalesce re-start.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Acked-by: Zheng Li <zheng.li@citrix.com>

GitHub: closes #8 on xapi-project/sm