docs: virt: convert UML documentation to ReST

Despite being an old document, it contains lots of information
that could still be useful.

The document has a nice style with makes easy to convert to
ReST. So, let's convert it to ReST.

This patch does:

	- Use proper markups for titles;
	- Mark and proper indent literal blocks;
	- don't use an 'o' character for lists;
	- other minor changes required for the doc to be parsed.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virt/index.rst

@@ -8,6 +8,7 @@ Linux Virtualization Support
    :maxdepth: 2
 
    kvm/index
+   uml/user_mode_linux
    paravirt_ops
 
 .. only:: html and subproject