docs: virt: Convert msr.txt to ReST format

- Use document title markup;
- Convert tables;
- Add blank lines and adjust indentation.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virt/kvm/index.rst

@@ -10,6 +10,7 @@ KVM
    amd-memory-encryption
    cpuid
    halt-polling
+   msr
    vcpu-requests
 
    arm/index