docs: return paragraph must be last

docs: return paragraph must be last

Commit eb70ceb tried to create a code block for
libvirt-libvirt.html#virConnectGetType, but failed to note
that our doc generator treats everything after "Returns" as
part of the return description rather than looking for
paragraph and code layout.  Fix some other API that also had
generic details crammed into the return type paragraph.

* src/libvirt.c (virConnectOpen, virConnectOpenReadOnly)
(virConnectOpenAuth, virConnectListAllDomains): Fit doc pattern.

Signed-off-by: Eric Blake <eblake@redhat.com>