KVM: SVM: Tag sev_supported_vmsa_features as read-only after init

KVM: SVM: Tag sev_supported_vmsa_features as read-only after init

Tag sev_supported_vmsa_features with __ro_after_init as it's configured by
sev_hardware_setup() and never written after initial configuration (and if
it were, that'd be a blatant bug).

Opportunistically relocate the variable out of the module params area now
that sev_es_debug_swap_enabled is gone (which largely motivated its
original location).

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://patch.msgid.link/20260109033101.1005769-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>