KVM: SVM: Provide helpers to set the error code

Provide helpers to set the error code when converting VMGEXIT SW_EXITINFO1 and
SW_EXITINFO2 codes from plain numbers to proper defines. Add comments for
better code readability.

No functionality changed.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Melody Wang <huibo.wang@amd.com>
Link: https://lore.kernel.org/r/20250225213937.2471419-3-huibo.wang@amd.com


[sean: tweak comments, fix formatting goofs]
Signed-off-by: Sean Christopherson <seanjc@google.com>