KVM: selftests: Add a test for gPAT handling in L2

When KVM_X86_QUIRK_NESTED_SVM_SHARED_PAT is disabled, verify that KVM
correctly virtualizes the host PAT MSR and the guest PAT register for
nested SVM guests.

With nested NPT disabled:
 * L1 and L2 share the same PAT
 * The vmcb12.g_pat is ignored

With nested NPT enabled:
 * An invalid g_pat in vmcb12 causes VMEXIT_INVALID
 * RDMSR(IA32_PAT) from L2 returns the value of the guest PAT register
 * WRMSR(IA32_PAT) from L2 is reflected in vmcb12's g_pat on VMEXIT
 * RDMSR(IA32_PAT) from L1 returns the value of the host PAT MSR

Verify that save/restore with the vCPU in guest mode behaves as expected in
both cases, e.g. preserves both hPAT and gPAT when NPT is enabled.

Originally-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Yosry Ahmed <yosry@kernel.org>
[sean: use even fancier macro shenanigans]
Link: https://patch.msgid.link/20260528231052.404737-1-seanjc@google.com
[sean: avoid use of goto, print skips]
Signed-off-by: Sean Christopherson <seanjc@google.com>

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