KVM: VMX: Allow INVPCID in guest without PCID

Remove the restriction that prevents VMX from exposing INVPCID to the
guest without PCID also being exposed to the guest.  The justification of
the restriction is that INVPCID will #UD if it's disabled in the VMCS.
While that is a true statement, it's also true that RDTSCP will #UD if
it's disabled in the VMCS.  Neither of those things has any dependency
whatsoever on the guest being able to set CR4.PCIDE=1, which is what is
effectively allowed by exposing PCID to the guest.

Removing the bogus restriction aligns VMX with SVM, and also allows for
an interesting configuration.  INVPCID is that fastest way to do a global
TLB flush, e.g. see native_flush_tlb_global().  Allowing INVPCID without
PCID would let a guest use the expedited flush while also limiting the
number of ASIDs consumed by the guest.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210212003411.1102677-4-seanjc@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>