pool_create: Resolve test errors

Originally posted to virt-test before the commit as PR 1236:

https://github.com/autotest/virt-test/pull/1236

This replaces and resolves issues mentioned from review. The main
difference between this version and the last one posted previously
is that the code now handles the transient 'default' storage pool
not being started prior to the test being run in the "error" case.
I also cleaned up and commented more the finally logic.