util/qemu-option: Document the get_opt_value() function

util/qemu-option: Document the get_opt_value() function

Coverity noticed commit 950c4e6c94 introduced a dereference before
null check in get_opt_value (CID1391003):

  In get_opt_value: All paths that lead to this null pointer
  comparison already dereference the pointer earlier (CWE-476)

We fixed this in commit 6e3ad3f0e31, but relaxed the check in commit
0c2f6e7ee99 because "No callers of get_opt_value() pass in a NULL
for the 'value' parameter".

Since this function is publicly exposed, it risks new users to do
the same error again. Avoid that documenting the 'value' argument
must not be NULL.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20200629070858.19850-1-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>