add host_device format description to qemu-img manpage

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: NAdrian Bridgett <adrian@bitcube.co.uk>
Signed-off-by: NAdrian Bridgett <adrian@bitcube.co.uk>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

qemu-img.texi

@@ -37,6 +37,12 @@ Linux or NTFS on Windows), then only the written sectors will reserve
 space. Use @code{qemu-img info} to know the real size used by the
 image or @code{ls -ls} on Unix/Linux.
 
+@item host_device
+
+Host device format. This format should be used instead of raw when
+converting to block devices or other devices where "holes" are not
+supported.
+
 @item qcow2
 QEMU image format, the most versatile format. Use it to have smaller
 images (useful if your filesystem does not supports holes, for example