docs: kvm: convert devices/vcpu.txt to ReST

- Use title markups;
- adjust indentation and add blank lines as needed;
- adjust tables to match ReST accepted formats;
- use :field: markups;
- mark code blocks as such.

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