KVM: x86: nSVM: Save gPAT to vmcb12.g_pat on VMEXIT

KVM: x86: nSVM: Save gPAT to vmcb12.g_pat on VMEXIT

According to the APM volume 3 pseudo-code for "VMRUN," when nested paging
is enabled in the vmcb, the guest PAT register (gPAT) is saved to the vmcb
on emulated VMEXIT.

When KVM_X86_QUIRK_NESTED_SVM_SHARED_PAT is disabled and the vCPU is in
guest mode with nested NPT enabled, save the vmcb02 g_pat field to the
vmcb12 g_pat field on emulated VMEXIT.

Fixes: 15038e147247 ("KVM: SVM: obey guest PAT")
Signed-off-by: Jim Mattson <jmattson@google.com>
Link: https://patch.msgid.link/20260407190343.325299-7-jmattson@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>