kvm/hyperv: add synic feature to CPU only if its not enabled

We need to make sure that synic CPU feature is not already enabled. If it is,
trying to enable it again will result in the following assertion:

Unexpected error in object_property_try_add() at ../qom/object.c:1268:
qemu-system-x86_64: attempt to add duplicate property 'synic' to object (type 'host-x86_64-cpu')

So enable synic only if its not enabled already.

Signed-off-by: Ani Sinha <anisinha@redhat.com>
Link: https://lore.kernel.org/r/20260225035000.385950-27-anisinha@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>