KVM: VMX: add support for switching of PERF_GLOBAL_CTRL

Some cpus have special support for switching PERF_GLOBAL_CTRL msr.
Add logic to detect if such support exists and works properly and extend
msr switching code to use it if available. Also extend number of generic
msr switching entries to 8.

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