KVM: SEV: Ignore MMIO requests of length '0'

KVM: SEV: Ignore MMIO requests of length '0'

Explicitly ignore MMIO requests of length '0', so that setting up the
software scratch area (and other code) doesn't have to worry about
underflowing the length, and to allow for special casing '0' in the
future.

Fixes: 8f423a80d299 ("KVM: SVM: Support MMIO for an SEV-ES guest")
Cc: stable@vger.kernel.org
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20260501202250.2115252-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>