xen 4.6 regression: xl create fails if domU have custom vifname

[Fabio Fantoni <fabio.fantoni@m2r.biz>]

Today trying xen 4.6.0-rc2 with all things needed for future production 
server I found a regression: xl create fails if domU have custom vifname.
xl create test.cfg
Parsing config from test.cfg
libxl: error: libxl_exec.c:118:libxl_report_child_exitstatus: 
/etc/xen/scripts/vif-bridge add [14581] exited with error status 1
libxl: error: libxl_device.c:1085:device_hotplug_child_death_cb: script: 
ip link set vif14.0-emu name frc-0-0-emu failed
libxl: error: libxl_create.c:1380:domcreate_attach_vtpms: unable to add 
nic devices
libxl: error: libxl_exec.c:118:libxl_report_child_exitstatus: 
/etc/xen/scripts/block remove [14615] exited with error status 1
libxl: error: libxl_device.c:1085:device_hotplug_child_death_cb: script: 
/etc/xen/scripts/block failed; error detected.
libxl: error: libxl.c:1580:libxl__destroy_domid: non-existant domain 14
libxl: error: libxl.c:1538:domain_destroy_callback: unable to destroy 
guest with domid 14
libxl: error: libxl.c:1465:domain_destroy_cb: destruction of domain 14 
failed

 From domU xl cfg:
vif=['bridge=xenbr0,mac=00:16:3e:dd:e3:f1,vifname=frc-0-0']

Same domU removing only vifname is working.

On production server with xen 4.4.2 or 4.5.1 domUs with vifname are working.

I did a fast search without found the exact bug/regression cause.

If you need more informations/tests tell me and I'll post them.

Thanks for any reply and sorry for my bad english.