docs/virt/kvm: Document configuring and running nested guests

This is a rewrite of this[1] Wiki page with further enhancements.  The
doc also includes a section on debugging problems in nested
environments, among other improvements.

[1] https://www.linux-kvm.org/page/Nested_Guests



Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>
Message-Id: <20200505112839.30534-1-kchamart@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>