Add whole disk image creation command to Installation Guide

Installation Guide lacks whole disk user image creation guidelines.
To use agent drivers, we should use images created with the parameter
"vm", like: "disk-image-create ubuntu vm dhcp-all-interfaces -o my-image".

Change-Id: I82c30c20a1e1280beb8d30bfd3dd12df917172cb
Co-Authored-By: Lilia Sampaio <liliars@lsd.​ufcg.​edu.​br>
Closes-Bug: #1514955

doc/source/deploy/install-guide.rst

@@ -621,15 +621,20 @@ them to the Image service:
    - Build the image your users will run (Ubuntu image has been taken as
      an example)::
 
-       disk-image-create ubuntu baremetal dhcp-all-interfaces grub2 -o my-image
+       Partition images:
+           disk-image-create ubuntu baremetal dhcp-all-interfaces grub2 -o my-image
 
-     The above command creates *my-image.qcow2*, *my-image.vmlinuz* and
-     *my-image.initrd* files. If you want to use Fedora image, replace
-     *ubuntu* with *fedora* in the above command. The *grub2* element is
-     only needed if local boot will be used to deploy *my-image.qcow2*,
-     otherwise the images *my-image.vmlinuz* and *my-image.initrd*
-     will be used for PXE booting after deploying the bare metal with
-     *my-image.qcow2*.
+       Whole disk images:
+           disk-image-create ubuntu vm dhcp-all-interfaces grub2 -o my-image
+
+     The partition image command creates *my-image.qcow2*, *my-image.vmlinuz* and
+     *my-image.initrd* files. The *grub2* element in the partition image creation
+     command is only needed if local boot will be used to deploy *my-image.qcow2*,
+     otherwise the images *my-image.vmlinuz* and *my-image.initrd* will be used for
+     PXE booting after deploying the bare metal with *my-image.qcow2*.
+
+     If you want to use Fedora image, replace *ubuntu* with *fedora* in the chosen
+     command.
 
    - To build the deploy image take a look at the `Building or
      downloading a deploy ramdisk image`_ section.