util: move virNetDevLinkDump to virnetlink.c

util: move virNetDevLinkDump to virnetlink.c

virNetDevLinkDump should have been in virnetlink.c, but that file
didn't exist yet when the function was created. It didn't really
matter until now - I found that having virnetlink.h included by
virnetdev.h caused build problems when trying to #include virnetdev.h
in a .c file in src/conf (due to missing directory in -I). Rather than
fix that to further institutionalize the incorrect placement of this
one function, this patch moves the function.