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>

Documentation/virtual/paravirt_ops.txt → Documentation/virtual/paravirt_ops.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+============
 Paravirt_ops
 ============
 
@@ -18,15 +21,15 @@ at boot time.
 pv_ops operations are classified into three categories:
 
 - simple indirect call
-  These operations correspond to high level functionality where it is
-  known that the overhead of indirect call isn't very important.
+   These operations correspond to high level functionality where it is
+   known that the overhead of indirect call isn't very important.
 
 - indirect call which allows optimization with binary patch
-  Usually these operations correspond to low level critical instructions. They
-  are called frequently and are performance critical. The overhead is
-  very important.
+   Usually these operations correspond to low level critical instructions. They
+   are called frequently and are performance critical. The overhead is
+   very important.
 
 - a set of macros for hand written assembly code
-  Hand written assembly codes (.S files) also need paravirtualization
-  because they include sensitive instructions or some of code paths in
-  them are very performance critical.
+   Hand written assembly codes (.S files) also need paravirtualization
+   because they include sensitive instructions or some of code paths in
+   them are very performance critical.