doc/_ext: print "default" using "literal" filter

doc/_ext: print "default" using "literal" filter

there is chance that the default value of an option is empty, before
this change the rendered result would be "````", and sphinx is confused
at seeing this:

Unexpected section title or transition.

as it takes it as a notion of section title. after this change,
the "literal" filter is always applied to the default value, so an empty
string is rendered like "<empty string>" instead.

Signed-off-by: Kefu Chai <kchai@redhat.com>