virutil: Provide non-linux impl for virGetFCHostNameByFabricWWN

virutil: Provide non-linux impl for virGetFCHostNameByFabricWWN

Currently, there's only linux implementation for
virGetFCHostNameByFabricWWN(). Since the symbol is exported in
our private symbols we ought to have implementation for other
platforms too. This also triggers compilation error on FreeBSD:

../src/.libs/libvirt_driver_storage_impl.a(libvirt_driver_storage_impl_la-storage_backend_scsi.o): In function `createVport':
/usr/home/jenkins/libvirt-master/systems/libvirt-freebsd/build/src/../../src/storage/storage_backend_scsi.c:740: undefined reference to `virGetFCHostNameByFabricWWN'

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>