VMware: add default image metadata for hw_vif_model

During glance image-create we are currently setting a null value
for the hw_vif_model, if we are unable to introspect from the vmdk
file.

This patch adds a default value if not found.

Change-Id: I6d0f551b18f03e2a799ae14c3347652a285693f0
Merge "VMware: add default image metadata for hw_vif_model"
Merge branch 'master' of https://github.com/openstack-dev/devstack