docs: kvm: devices/arm-vgic-its.txt to ReST format

- Fix document title to match ReST format
- Convert the table to be properly recognized
- use proper markups for literal blocks
- Some indentation fixes to match ReST

While here, add an index for kvm devices.

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