docs: kvm: devices/arm-vgit-v3.txt to ReST

- Use title markups;
- change indent to match ReST syntax;
- use proper table markups;
- use literal block markups.

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