iotests: Add test 221 to catch qemu-img map regression

iotests: Add test 221 to catch qemu-img map regression

Although qemu-img creates aligned files (by rounding up), it
must also gracefully handle files that are not sector-aligned.
Test that the bug fixed in the previous patch does not recur.

It's a bit annoying that we can see the (implicit) hole past
the end of the file on to the next sector boundary, so if we
ever reach the point where we report a byte-accurate size rather
than our current behavior of always rounding up, this test will
probably need a slight modification.

Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>