pci: add helper functions to check ranges overlap.

add helper function to check ranges overlap suggested by
Michael S. Tsirkin <mst@redhat.com>.
His original suggestion was to use [first, last], however I chosen
to use offset, length pair, i.e. [offset, offset + length)
because pci configuration space related functions all uses offset and length
pair, so it helps to avoid to type constant twice.

Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
