docs: kvm: Convert timekeeping.txt to ReST format

- Use document title and chapter markups;
- Add markups for literal blocks;
- Add markups for tables;
- use :field: for field descriptions;
- Add blank lines and adjust indentation.

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

Author: mchehab

Documentation/virt/kvm/index.rst

@@ -18,6 +18,7 @@ KVM
    nested-vmx
    ppc-pv
    s390-diag
+   timekeeping
    vcpu-requests
 
    arm/index