The xen-hvm hypervisor has two extra hvparams concerning NICS;
nic_type, and vif_type. Let _WriteNicConfig() calculate all needed
extra options for the instance's NICs based on its hvparams.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>