KVM: SVM: support writing 0 to K8 performance counter control registers

This lets SVM ignore writes of the value 0 to the performance counter control
registers.  Thus enabling them will still fail in the guest, but a write of 0
which keeps them disabled is accepted.  This is required to boot Windows
Vista 64bit.

[avi: avoid fall-thru in switch statement]

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Markus Rechberger <markus.rechberger@amd.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>