KVM: SEV: expose KVM_CAP_VM_MOVE_ENC_CONTEXT_FROM capability

The capability, albeit present, was never exposed via KVM_CHECK_EXTENSION.

Fixes: b5663931

 ("KVM: SEV: Add support for SEV intra host migration")
Cc: Peter Gonda <pgonda@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>