KVM: VMX: Enable MSR-BASED TPR shadow even if APICv is inactive

I observed that kvmvapic(to optimize flexpriority=N or AMD) is used
to boost TPR access when testing kvm-unit-test/eventinj.flat tpr case
on my haswell desktop (w/ flexpriority, w/o APICv). Commit (8d14695f9542
x86, apicv: add virtual x2apic support) disable virtual x2apic mode
completely if w/o APICv, and the author also told me that windows guest
can't enter into x2apic mode when he developed the APICv feature several
years ago. However, it is not truth currently, Interrupt Remapping and
vIOMMU is added to qemu and the developers from Intel test windows 8 can
work in x2apic mode w/ Interrupt Remapping enabled recently.

This patch enables TPR shadow for virtual x2apic mode to boost
windows guest in x2apic mode even if w/o APICv.

Can pass the kvm-unit-test.

Suggested-by: Radim Krčmář <[email protected]>
Suggested-by: Wincy Van <[email protected]>
Reviewed-by: Radim Krčmář <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Radim Krčmář <[email protected]>
Cc: Wincy Van <[email protected]>
Cc: Yang Zhang <[email protected]>
Signed-off-by: Wanpeng Li <[email protected]>
Signed-off-by: Radim Krčmář <[email protected]>

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