util: Introduce virBufferAddBuffer

util: Introduce virBufferAddBuffer

This API joins the following two lines:

char *s = virBufferContentAndReset(buf1);
virBufferAdd(buf2, s, -1);

into one:

virBufferAddBuffer(buf2, buf1);

With one exception: there's no re-indentation applied to @buf1.
The idea is, that in general both can have different indentation
(like the test I'm adding proves)

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>