Documentation: virtual: Convert paravirt_ops.txt to .rst

Convert paravirt_opts.txt to .rst format to be able to be parsed by
sphinx.

Made some minor spacing and formatting corrections to make defintions
much more clear and easy to read. Added default kernel license to the
document.

Signed-off-by: Luke Nowakowski-Krijger <lnowakow@eng.ucsd.edu>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>