iotests: use TEST_IMG_FILE instead of TEST_IMG in _require_large_file

iotests: use TEST_IMG_FILE instead of TEST_IMG in _require_large_file

We need to check that we are able to create large enough file which is
used as an export base rather than connection URL. Unfortunately, there
are cases when the TEST_IMG_FILE is not defined. We should fallback to
TEST_IMG in that case.

This problem has been detected when running
    ./check -nbd 5
The test should be able to run while it does not.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Hanna Reitz <hreitz@redhat.com>
CC: Eric Blake <eblake@redhat.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-ID: <20230906140917.559129-2-den@openvz.org>
Tested-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>