Rename the existing --current flag to the new name --active,
while adding a new flag --current to expose the new
VIR_DOMAIN_AFFECT_CURRENT flag of virDomainGetVcpusFlags.

For backwards compability, the output does not change (even
though the label "current" no longer matches the spelling of
the option that would trigger that number in isolation), and
we accept "--current --live" as an undocumented synonym for
"--active --live" to avoid breaking any existing clients.

* tools/virsh.c (cmdVcpucount): Add --active flag, and rearrange
existing flag handling to expose VIR_DOMAIN_AFFECT_CURRENT support.
* tools/virsh.pod (vcpucount): Document this.