core: Fix compareTo for SRV records

This suggested implemenation is more natural as it is based
on substraction of two values which is commonly used in comparisson

Bug-Url: https://bugzilla.redhat.com/985623
Change-Id: Iff93a85f529633934515015653c517500bf51c6b
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>