Properly track VBD and VDI connections in xenapi fake
Properly track VBD and VDI connections in xenapi fake

The fake xenapi module correctly destroys VBDs when destroying a VM
but it won't destroy VBDs when destroying a VDI. If an exception
occurs during a spawn when running test_xenapi, then another exception
can occur which obscures the original (and likely more important)
exception. This can make troubleshooting test suite failures more
difficult.

Change-Id: I19aad7f39b0707893a04784faef90815f7b0effb

n0ano/gantt,n0ano/gantt