virttest.libvirt_xml.vm_xml: Add support of change current vcpu

Support change current vcpu in set_vm_vcpus method, add new
current_vcpu XMLAttribute in VMXMLBase.

This will benefit vcpu hotplug for setting current vcpu first.

Signed-off-by: Wayne Sun <gsun@redhat.com>
libvirt.tests: Add virsh vcpucount test

The test works for domain state as "shut off" or "running", it check
vcpucount result after vcpu hotplug using setvcpus.

For setvcpus, include four valid options:
  --config
  --config --maximum
  --live
  --guest

For vcpucount options, restrict up to 2 options together, upstream libvirt
support more options combinations now (e.g. 3 options together or single
--maximum option), for backward support, only following options are
checked:
  None
  --config --active
  --config --maximum
  --live --active
  --live --maximum
  --current --active
  --current --maximum
  --guest

Signed-off-by: Wayne Sun <gsun@redhat.com>