xenapi: max out VCPU count

Give as much VCPUs to the DevStack machine as possible. First asking
xenapi about its CPU count, and as a fallback, count the CPUs in dom0.
This should result in faster test runs.

Change-Id: I1ffb99ecd435f1d7eb5754fe9cd99f0e8ceae6dc
Merge "xenapi: max out VCPU count"