[POWERPC] Don't access HID registers if running on a Hypervisor.

The following patch avoids accessing Hypervisor privilege HID
registers when running on a Hypervisor (MSR[HV]=0).

Signed-off-by: Jimi Xenidis <jimix@watson.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>