create_box.sh: fix IMG_SIZE parsing on Debian Sid
On Debian Sid IMG_SIZE becomes "2\n20" (which makes vagrant box add
consider the resulting .box corrupt) due to "virtual-size" occurring
twice, a second unexpected time as a "child" for whatever reason. So
let's ignore that second "virtual-size" by taking indentation into
account.
Refs: #19927 (closed)
Closes #19927 (closed)