The arguments of error_setg() & friends should yield a short error
string without newlines.

Two places try to append additional help to the error message by
embedding newlines in the error string.  That's nice, but let's do it
the right way, with error_append_hint().

Cc: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1450452927-8346-20-git-send-email-armbru@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>