libvirt driver: set driver name consistently

Add pick_disk_driver_name to utils, to be able to set driver_name
consistently across volume and imagebackend. This is mostly required for
xen.

Xen requires specifying both the driver name and type attributes when
defining disks.  E.g. the following disk XML snippet

  <disk type='file' device='disk'>
    <driver type='qcow2'/>
    <source file='/tmp/disk0.qcow2'/>
    <target dev='xvda'/>
  </disk>

produces incorrect configuration in the xen tools

  (uname file:/tmp/disk0.qcow2)

Including the driver name fixes this

  <disk type='file' device='disk'>
    <driver name='tap' type='qcow2'/>
    <source file='/tmp/disk0.qcow2'/>
    <target dev='xvda'/>
  </disk>

produces

  (uname tap:qcow2:/tmp/disk0.qcow2)

Change-Id: Id984e0c431b6fc743a39e6e6f3dc3eb289fbb43a

0 files changed, 0 insertions, 0 deletions