KVM: VMX: Don't intercept MSR_KERNEL_GS_BASE

Windows 2008 accesses this MSR often on context switch intensive workloads;
since we run in guest context with the guest MSR value loaded (so swapgs can
work correctly), we can simply disable interception of rdmsr/wrmsr for this
MSR.

A complication occurs since in legacy mode, we run with the host MSR value
loaded. In this case we enable interception.  This means we need two MSR
bitmaps, one for legacy mode and one for long mode.

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