libxl: Allow libxl to set NIC devid

libxl contains logic to determine an appropriate devid for new devices
that do not specify one in their configuration.  For all device types
except NICs, the libxl driver allows libxl to determine devid.  Do the
same for NICs.

src/libxl/libxl_conf.c

@@ -609,8 +609,6 @@ libxlMakeNicList(virDomainDefPtr def,  libxl_domain_config *d_config)
     }
 
     for (i = 0; i < nnics; i++) {
-        x_nics[i].devid = i;
-
         if (libxlMakeNic(l_nics[i], &x_nics[i]))
             goto error;
     }