KVM: VMX: Mark VCPU_EXREG_CR3 dirty when !CR0_PG -> CR0_PG if EPT + !URG

When !CR0_PG -> CR0_PG, vcpu->arch.cr3 becomes active, but GUEST_CR3 is
still vmx->ept_identity_map_addr if EPT + !URG.  So VCPU_EXREG_CR3 is
considered to be dirty and GUEST_CR3 needs to be updated in this case.

Reported-by: Maxim Levitsky <mlevitsk@redhat.com>
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Message-Id: <20211216021938.11752-4-jiangshanlai@gmail.com>
Fixes: c62c7bd4

 ("KVM: VMX: Update vmcs.GUEST_CR3 only when the guest CR3 is dirty")
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>