iotests/138: Test leaks/corruptions fixed report

iotests/138: Test leaks/corruptions fixed report

Test that qemu-img check reports the number of leaks and corruptions
fixed in its JSON report (after a successful run).

While touching the _unsupported_imgopts line, adjust the note on why
data_file does not work with this test: The current comment sounds a bit
like it is a mistake for qemu-img check not to check external data
files' refcounts.  But there are no such refcounts, so it is no mistake.
Just say that qemu-img check does not do much for external data files,
and this is why this test does not work with them.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200324172757.1173824-4-mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>