KVM: x86/pmu: do not mask the value that is written to fixed PMUs

KVM: x86/pmu: do not mask the value that is written to fixed PMUs

According to the SDM, for MSR_IA32_PERFCTR0/1 "the lower-order 32 bits of
each MSR may be written with any value, and the high-order 8 bits are
sign-extended according to the value of bit 31", but the fixed counters
in real hardware are limited to the width of the fixed counters ("bits
beyond the width of the fixed-function counter are reserved and must be
written as zeros").  Fix KVM to do the same.

Reported-by: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>