xen:pvhvm: enable PVHVM VCPU placement when using more than 32 CPUs.

PVHVM running with more than 32 vcpus and pv_irq/pv_time enabled
need VCPU placement to work, or else it will softlockup.

CC: stable@kernel.org
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>