KVM: SEV: Disallow LAUNCH_FINISH if vCPUs are actively being created

KVM: SEV: Disallow LAUNCH_FINISH if vCPUs are actively being created

Reject LAUNCH_FINISH for SEV-ES and SNP VMs if KVM is actively creating
one or more vCPUs, as KVM needs to process and encrypt each vCPU's VMSA.
Letting userspace create vCPUs while LAUNCH_FINISH is in-progress is
"fine", at least in the current code base, as kvm_for_each_vcpu() operates
on online_vcpus, LAUNCH_FINISH (all SEV+ sub-ioctls) holds kvm->mutex, and
fully onlining a vCPU in kvm_vm_ioctl_create_vcpu() is done under
kvm->mutex.  I.e. there's no difference between an in-progress vCPU and a
vCPU that is created entirely after LAUNCH_FINISH.

However, given that concurrent LAUNCH_FINISH and vCPU creation can't
possibly work (for any reasonable definition of "work"), since userspace
can't guarantee whether a particular vCPU will be encrypted or not,
disallow the combination as a hardening measure, to reduce the probability
of introducing bugs in the future, and to avoid having to reason about the
safety of future changes related to LAUNCH_FINISH.

Cc: Jethro Beekman <jethro@fortanix.com>
Closes: https://lore.kernel.org/all/b31f7c6e-2807-4662-bcdd-eea2c1e132fa@fortanix.com
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260310234829.2608037-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>