xen/vcpu: Use a unified name about cpu hotplug state for pv and pvhvm

As xen_cpuhp_setup is called by PV and PVHVM, the name of "x86/xen/hvm_guest"
is confusing.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>