KVM: SVM: Condition sev_enabled and sev_es_enabled on CONFIG_KVM_AMD_SEV=y

Define sev_enabled and sev_es_enabled as 'false' and explicitly #ifdef
out all of sev_hardware_setup() if CONFIG_KVM_AMD_SEV=n.  This kills
three birds at once:

  - Makes sev_enabled and sev_es_enabled off by default if
    CONFIG_KVM_AMD_SEV=n.  Previously, they could be on by default if
    CONFIG_AMD_MEM_ENCRYPT_ACTIVE_BY_DEFAULT=y, regardless of KVM SEV
    support.

  - Hides the sev and sev_es modules params when CONFIG_KVM_AMD_SEV=n.

  - Resolves a false positive -Wnonnull in __sev_recycle_asids() that is
    currently masked by the equivalent IS_ENABLED(CONFIG_KVM_AMD_SEV)
    check in svm_sev_enabled(), which will be dropped in a future patch.

Reviewed by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210422021125.3417167-9-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>