xenFormatNet: correct `type=netfront' to 'type=vif' to match libxl

xenFormatNet: correct `type=netfront' to 'type=vif' to match libxl

According to current xl.cfg docs and xl codes, it uses type=vif
instead of type=netfront.

Currently after domxml-to-native, libvirt xml model=netfront will be
converted to xl type=netfront. This has no problem before, xen codes
for a long time just check type=ioemu, if not, set type to _VIF.

Since libxl uses parse_nic_config to avoid duplicate codes, it
compares 'type=vif' and 'type=ioemu' for valid parameters, others
are considered as invalid, thus we have problem with type=netfront
in xl config file.
 #xl create sles12gm-hvm.orig
 Parsing config from sles12gm-hvm.orig
 Invalid parameter `type'.

Correct the conversion in libvirt, so that it matchs libxl codes
and also xl.cfg.

Signed-off-by: Chunyan Liu <cyliu@suse.com>