libxl: fix vif.ifname when used with stub device model.

libxl: fix vif.ifname when used with stub device model.

Currently the same libxl_device_nic is used to create both the HVM domain and
its stub domain's NICs. This means that if a vifname is provided both the HVM
domains PV NIC and the stub domains PV NIC will get the same name and the
DM's NIC will fail to be attached.

Instead launder the libxl_device_nic to add the TAP_DEVICE_SUFFIX ("-emu").
This is a bit of a misnomer, since the device is actually PV, but it is used to
"back" the emulated device in the stub domain and this naming scheme is
consistent with the non-stub case and is known to work e.g. with our hotplug
scripts.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>