qemu-io: Optionally verify only part of read data

There are reasonable test cases where a read must span areas that are not
uniformly filled with one pattern but contains several parts. This makes -P
useless for them currently.

Introducing additional options which determine the part of the read data that
should be verified with the given pattern allows to check such reads.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
