Allow VIF model to be chosen per image

This allows for an image in glance to be annotated with a
property describing the required VIF model

eg

  # glance image-update \
           --property vif_model=e1000 \
           f16-x86_64-openstack-sda

Valid model values vary per the libvirt_type setting:

 qemu/kvm: 'virtio', 'ne2k_pci', 'pcnet', 'rtl8139', 'e1000'
 xen: 'netfront', 'ne2k_pci', 'pcnet', 'rtl8139', 'e1000'

Requesting an unsupported VIF model will cause the guest
instance to fail to launch.

DocImpact
Blueprint: libvirt-custom-hardware
Change-Id: Idbee0c61cffdb43db5668fb88869a5d30278593f
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>