KVM: SVM: Do not set sev->es_active until KVM_SEV_ES_INIT completes

Set sev->es_active only after the guts of KVM_SEV_ES_INIT succeeds.  If
the command fails, e.g. because SEV is already active or there are no
available ASIDs, then es_active will be left set even though the VM is
not fully SEV-ES capable.

Refactor the code so that "es_active" is passed on the stack instead of
being prematurely shoved into sev_info, both to avoid having to unwind
sev_info and so that it's more obvious what actually consumes es_active
in sev_guest_init() and its helpers.

Fixes: ad73109a ("KVM: SVM: Provide support to launch and run an SEV-ES guest")
Cc: stable@vger.kernel.org
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210331031936.2495277-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>