libvirt: fix virsh edit test.

* we can not assume the guest with 1 vcpu.
Change the pattern in sed from `1<\/vcpu>` to `[0-9]*<\/vcpu>`

* add a vcpucount function in virsh module. And use it in virsh_edit
to get the vcpucount of VM.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
libvirt: fix virsh_edit.py from pep8

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>