KVM: SEV: Rename kvm_ghcb_get_sw_exit_code() to kvm_get_cached_sw_exit_code()

KVM: SEV: Rename kvm_ghcb_get_sw_exit_code() to kvm_get_cached_sw_exit_code()

Rename kvm_ghcb_get_sw_exit_code() to kvm_get_cached_sw_exit_code() to make
it clear that KVM is getting the cached value, not reading directly from
the guest-controlled GHCB.  More importantly, vacating
kvm_ghcb_get_sw_exit_code() will allow adding a KVM-specific macro-built
kvm_ghcb_get_##field() helper to read values from the GHCB.

No functional change intended.

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20250919223258.1604852-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>