KVM: VMX: Use proper accessor to read guest CR4 in handle_desc()

Use kvm_is_cr4_bit_set() to read guest CR4.UMIP when sanity checking that
a descriptor table VM-Exit occurs if and only if guest.CR4.UMIP=1.  UMIP
can't be guest-owned, i.e. using kvm_read_cr4_bits() to decache guest-
owned bits isn't strictly necessary, but eliminating raw reads of
vcpu->arch.cr4 is desirable as it makes it easy to visually audit KVM for
correctness.

Opportunistically add a compile-time assertion that UMIP isn't guest-owned
as letting the guest own UMIP isn't compatible with emulation (or any CR4
bit that is emulated by KVM).

Opportunistically change the WARN_ON() to a ONCE variant.  When the WARN
fires, it fires _a lot_, and spamming the kernel logs ends up doing more
harm than whatever led to KVM's unnecessary emulation.
Reported-by: Robert Hoo <robert.hu@intel.com>
Link: https://lore.kernel.org/all/20230310125718.1442088-4-robert.hu@intel.com
Link: https://lore.kernel.org/r/20230413231914.1482782-3-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/vmx/vmx.c

@@ -5402,7 +5402,13 @@ static int handle_set_cr4(struct kvm_vcpu *vcpu, unsigned long val)
 
 static int handle_desc(struct kvm_vcpu *vcpu)
 {
-	WARN_ON(!(vcpu->arch.cr4 & X86_CR4_UMIP));
+	/*
+	 * UMIP emulation relies on intercepting writes to CR4.UMIP, i.e. this
+	 * and other code needs to be updated if UMIP can be guest owned.
+	 */
+	BUILD_BUG_ON(KVM_POSSIBLE_CR4_GUEST_BITS & X86_CR4_UMIP);
+
+	WARN_ON_ONCE(!kvm_is_cr4_bit_set(vcpu, X86_CR4_UMIP));
 	return kvm_emulate_instruction(vcpu, 0);
 }