Add a test for qcow2 copy-on-read behavior, including exposure
for the just-fixed bugs.

The copy-on-read behavior is always to a qcow2 image, but the
test is careful to allow running with most image protocol/format
combos as the backing file being copied from (luks being the
exception, as it is harder to pass the right secret to all the
right places).  In fact, for './check nbd', this appears to be
the first time we've had a qcow2 image wrapping NBD, requiring
an additional line in _filter_img_create to match the similar
line in _filter_img_info.

Invoking blkdebug to prove we don't write too much took some
effort to get working; and it requires that $TEST_WRAP (based
on $TEST_DIR) not be subject to word splitting.  We may decide
later to have the entire iotests suite use relative rather than
absolute names, to avoid problems inherited by the absolute
name of $PWD or $TEST_DIR, at which point the sanity check in
this commit could be simplified.

This test requires at least 2G of consecutive memory to succeed;
as such, it is prone to spurious failures, particularly on
32-bit machines under load.  This situation is detected and
triggers an early exit to skip the test, rather than a failure.
To manually provoke this setup on a beefier machine, I used:
  $ (ulimit -S -v 1000000; ./check -qcow2 197)
Signed-off-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>