Add a 'hw_' namespace to glance hardware config properties

This updates the libvirt driver to use a 'hw_' prefix on
the names of hardware config properties it looks up for
glance images. Thus the new syntax for setting custom
hardware properties is

      # glance image-update \
               --property hw_disk_bus=ide \
               --property hw_cdrom_bus=ide \
               --property hw_vif_model=e1000 \
               f16-x86_64-openstack-sda

There is no back-compat support for the previous 'disk_bus',
'cdrom_bus' and 'vif_model' names, since they were never
made available in any formal release, only development
snapshots.

DocImpact
Blueprint: libvirt-custom-hardware
Fixes bug 1132739
Change-Id: Ic15ef0eba065113b35bbb2713be2532c858c3534
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Merge "Add a 'hw_' namespace to glance hardware config properties"