KVM: SVM: Add a helper to detect VMRUN failures

Add a helper to detect VMRUN failures so that KVM can guard against its
own long-standing bug, where KVM neglects to set exitcode[63:32] when
synthesizing a nested VMFAIL_INVALID VM-Exit.  This will allow fixing
KVM's mess of treating exitcode as two separate 32-bit values without
breaking KVM-on-KVM when running on an older, unfixed KVM.

Cc: Jim Mattson <jmattson@google.com>
Cc: Yosry Ahmed <yosry.ahmed@linux.dev>
Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://patch.msgid.link/20251230211347.4099600-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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