Fix concurrent deletes in virt driver

The _unprovision function used a class member to track the number
of calls to ironic to check the provision state. When more than one
delete is happening concurrently, the deletes will both add to the
same counter. This commit fixes this issue to use a local counter.

Partial-Bug: #1326364

Change-Id: I2552332445fe0e614970ec28eddf7c1ae94e34be
Merge "Fix concurrent deletes in virt driver"