libvirt-domain: Fix documentation of virDomainInterfaceAddresses()

libvirt-domain: Fix documentation of virDomainInterfaceAddresses()

Ever since of v1.2.14-rc1~105 the hwaddr member of
_virDomainInterface struct can be NULL. And this is documented
inside the struct. But then the public API documents it can never
be NULL which is obviously wrong. Fix the public API
documentation.

Fixes: 3640245db7d72bf8e05df726587625a6328c895e
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>