Allow disk driver to be chosen per image

This allows for an image in glance to be annotated with a
property describing the required disk / cdrom bus type.

eg

  # glance image-update \
           --property disk_bus=ide \
           --property cdrom_bus=ide \
           f16-x86_64-openstack-sda

Valid bus values vary per the libvirt_type setting:

 qemu/kvm: 'virtio', 'scsi', 'ide', 'usb'
 xen: 'xen', 'ide'
 uml: 'uml'

Requesting an unsupported disk bus will cause the guest
instance to fail to launch.

DocImpact
Blueprint: libvirt-custom-hardware
Change-Id: Ibc20b843ceccf374eae246384a3f246cc1c52a28
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Merge "Allow disk driver to be chosen per image"