KVM: do not #GP on perf MSR writes when vPMU is disabled

Return to behaviour perf MSR had before introducing vPMU in case vPMU
is disabled. Some guests access those registers unconditionally and do
not expect it to fail.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>