docs: kvm: Convert api.txt to ReST format

convert api.txt document to ReST format while trying to keep
its format as close as possible with the authors intent, and
avoid adding uneeded markups.

- Use document title and chapter markups;
- Convert tables;
- Add markups for literal blocks;
- 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>

Documentation/virt/kvm/index.rst

@@ -7,6 +7,7 @@ KVM
 .. toctree::
    :maxdepth: 2
 
+   api
    amd-memory-encryption
    cpuid
    halt-polling