docs: convert xl-disk-configuration into a man page

Convert xl-disk-configuration.txt from plain text file to a POD file
to get it as a man page. The references to it in the other man pages
are also updated.

Signed-off-by: Cédric Bosdonnat <cbosdonnat@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>

docs/INDEX

@@ -16,7 +16,6 @@ misc/tscmode			TSC Mode HOWTO
 misc/vbd-interface		Xen Guest Disk (VBD) Interface
 misc/xenstore			Xenstore protocol specification
 misc/xenstore-paths		Xenstore path documentation
-misc/xl-disk-configuration	XL Disk Configuration
 misc/distro_mapping		Distro Directory Layouts
 misc/dump-core-format		Xen Core Dump Format
 misc/vtd			VT-d HOWTO