KVM: x86: Define KVM_X86_QUIRK_NESTED_SVM_SHARED_PAT

KVM: x86: Define KVM_X86_QUIRK_NESTED_SVM_SHARED_PAT

Define a quirk to control whether nested SVM shares L1's PAT with L2
(legacy behavior) or gives L2 its own independent gPAT (correct behavior
per the APM).

When the quirk is enabled (default), L2 shares L1's PAT, preserving the
legacy KVM behavior. When userspace disables the quirk, KVM correctly
virtualizes the PAT for nested SVM guests, giving L2 a separate gPAT as
specified in the AMD architecture.

Signed-off-by: Jim Mattson <jmattson@google.com>
Link: https://patch.msgid.link/20260407190343.325299-2-jmattson@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>