vlog: Make the most common module reference more direct.

vlog: Make the most common module reference more direct.

Most vlog calls are for the log module owned by the translation unit being
compiled, but this module was referenced indirectly through a pointer
variable.  That seems silly, so this commit changes the code so that the
local vlog module is referred to directly, as &this_module.

We could get rid of the global variables for vlog modules entirely, but
I like getting linker errors when there's a duplicate module name.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Russell Bryant <russell@ovn.org>