This patch factors out the vCPU count retrieval including fallback means
into vshCPUCountCollect() and removes the duplicated code to retrieve
individual counts.

The --current flag (this flag is assumed by default) now works also with
--maximum or --active without the need to explicitly specify the state
of the domain that is requested.

This patch also fixes the output of "virsh vcpucount domain" on inactive
domains:

Before:
$ virsh vcpucount domain
maximum      config         4
error: Requested operation is not valid: domain is not running
current      config         4
error: Requested operation is not valid: domain is not running

After:
$virsh vcpucount domain
maximum      config         4
current      config         4

.. and for transient domains too:

Before:
$ virsh vcpucount transient-domain
error: Requested operation is not valid: cannot change persistent config of a transient domain
maximum      live           3
error: Requested operation is not valid: cannot change persistent config of a transient domain
current      live           1

After:
$ virsh vcpucount transient-domain
maximum      live           3
current      live           1