Converting files using "qemu-img convert" onto logical volumes (or any
block device) you need to use the currently undocumented "host_device"
format.  This patch adds the required documentation to the manpage.

Reported-by: Adrian Bridgett <adrian@bitcube.co.uk>
Signed-off-by: Adrian Bridgett <adrian@bitcube.co.uk>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>