CA-120162: don't fail if the GC can't be kicked

When a VDI.delete needs to abort the GC and restart it, a deadlock might
occur which leads to reporting that VDI.delete while it has partially
succeeded, as the actual deletion of the VDI will happen sooner or
later. Fixing the deadlock is not straightforward, so the best effort
solution is to simply ignore such failures.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>