KVM: PMU: add proper support for fixed counter 2

Currently pmu emulation emulates fixed counter 2 as bus cycles
architectural counter, but since commit 9c1497ea perf has
pseudo encoding for it. Use it.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>