KVM: nVMX: Disable intercept for FS/GS base MSRs in vmcs02 when possible

If L1 is using an MSR bitmap, unconditionally merge the MSR bitmaps from
L0 and L1 for MSR_{KERNEL,}_{FS,GS}_BASE.  KVM unconditionally exposes
MSRs L1.  If KVM is also running in L1 then it's highly likely L1 is
also exposing the MSRs to L2, i.e. KVM doesn't need to intercept L2
accesses.

Based on code from Jintack Lim.

Cc: Jintack Lim <jintack@xxxxxxxxxxxxxxx>
Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>