KVM: SVM: Disallow guest from changing userspace's MSR_AMD64_DE_CFG value

KVM: SVM: Disallow guest from changing userspace's MSR_AMD64_DE_CFG value

Inject a #GP if the guest attempts to change MSR_AMD64_DE_CFG from its
*current* value, not if the guest attempts to write a value other than
KVM's set of supported bits.  As per the comment and the changelog of the
original code, the intent is to effectively make MSR_AMD64_DE_CFG read-
only for the guest.

Opportunistically use a more conventional equality check instead of an
exclusive-OR check to detect attempts to change bits.

Fixes: d1d93fa90f1a ("KVM: SVM: Add MSR-based feature support for serializing LFENCE")
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20240802181935.292540-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>