gnt-instance: new hypervisor parameter 'vif_type'

This patch fixes issue 247. It empowers the user to specify
explicitly the vif type of a nic configuration of a Xen HVM
instance. This includes the option of setting it to '' on the
commandline, causing Ganeti to leave out the 'type' parameter
in the vif configuration completely.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>