iotests: Simplify _filter_img_create() a bit

iotests: Simplify _filter_img_create() a bit

Not only is it a bit stupid to try to filter multi-line "Formatting"
output (because we only need it for a single test, which can easily be
amended to no longer need it), it is also problematic when there can be
output after a "Formatting" line that we do not want to filter as if it
were part of it.

So rename _filter_img_create to _do_filter_img_create, let it filter
only a single line, and let _filter_img_create loop over all input
lines, calling _do_filter_img_create only on those that match
/^Formatting/ (basically, what _filter_img_create_in_qmp did already).
(And fix 020 to work with that.)

Reported-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200709110205.310942-1-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>