KVM: SVM: Macrofy SEV=n versions of sev_xxx_guest()

KVM: SVM: Macrofy SEV=n versions of sev_xxx_guest()

Define sev_{,es_,snp_}guest() as "false" when SEV is disabled via Kconfig,
i.e. when CONFIG_KVM_AMD_SEV=n.  Despite the helpers being __always_inline,
gcc-12 is somehow incapable of realizing that the return value is a
compile-time constant and generates sub-optimal code.

Opportunistically clump the paths together to reduce the amount of
ifdeffery.

No functional change intended.

Link: https://lore.kernel.org/r/20241127234659.4046347-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>