KVM: VMX: Track common EPTP for Hyper-V's paravirt TLB flush

Explicitly track the EPTP that is common to all vCPUs instead of
grabbing vCPU0's EPTP when invoking Hyper-V's paravirt TLB flush.
Tracking the EPTP will allow optimizing the checks when loading a new
EPTP and will also allow dropping ept_pointer_match, e.g. by marking
the common EPTP as invalid.

This also technically fixes a bug where KVM could theoretically flush an
invalid GPA if all vCPUs have an invalid root.  In practice, it's likely
impossible to trigger a remote TLB flush in such a scenario.  In any
case, the superfluous flush is completely benign.

Reviewed-by: Vitaly Kuznetsov <[email protected]>
Signed-off-by: Sean Christopherson <[email protected]>
Signed-off-by: Sean Christopherson <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

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