KVM: x86: Invoke vendor's vcpu_after_set_cpuid() after all common updates

Move the call to kvm_x86_ops.vcpu_after_set_cpuid() to the very end of
kvm_vcpu_after_set_cpuid() to allow the vendor implementation to react
to changes made by the common code.  In the near future, this will be
used by VMX to update its CR4 guest/host masks to account for reserved
bits.  In the long term, SGX support will update the allowed XCR0 mask
for enclaves based on the vCPU's allowed XCR0.

vcpu_after_set_cpuid() (nee kvm_update_cpuid()) was originally added by
commit 2acf923e ("KVM: VMX: Enable XSAVE/XRSTOR for guest"), and was
called separately after kvm_x86_ops.vcpu_after_set_cpuid() (nee
kvm_x86_ops->cpuid_update()).  There is no indication that the placement
of the common code updates after the vendor updates was anything more
than a "new function at the end" decision.

Inspection of the current code reveals no dependency on kvm_x86_ops'
vcpu_after_set_cpuid() in kvm_vcpu_after_set_cpuid() or any of its
helpers.  The bulk of the common code depends only on the guest's CPUID
configuration, kvm_mmu_reset_context() does not consume dynamic vendor
state, and there are no collisions between kvm_pmu_refresh() and VMX's
update of PT state.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200930041659.28181-3-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>