KVM: Extended shared_msr_global to per CPU

shared_msr_global saved host value of relevant MSRs, but it have an
assumption that all MSRs it tracked shared the value across the different
CPUs. It's not true with some MSRs, e.g. MSR_TSC_AUX.

Extend it to per CPU to provide the support of MSR_TSC_AUX, and more
alike MSRs.

Notice now the shared_msr_global still have one assumption: it can only deal
with the MSRs that won't change in host after KVM module loaded.

Signed-off-by: Sheng Yang <sheng@linux.intel.com>
Signed-off-by: Avi Kivity <avi@redhat.com>