docs: kvm: convert devices/vm.txt to ReST

- Use title markups;
- adjust indentation and add blank lines as needed;
- use :field: markups;
- Use cross-references;
- mark code blocks as such.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virt/kvm/devices/index.rst

@@ -14,3 +14,4 @@ Devices
    s390_flic
    vcpu
    vfio
+   vm