iotests/291: Filter irrelevant parts of img-info

iotests/291: Filter irrelevant parts of img-info

We need to let _img_info emit the format-specific information so we get
the list of bitmaps we want, but we do not need anything but the
bitmaps.  So filter out everything that is irrelevant to us.  (Ideally,
this would be a generalized function in common.filters that takes a list
of things to keep, but that would require implementing an anti-bitmap
filter, which would be hard, and which we do not need here.  So that is
why this function is just a local hack.)

This lets 291 pass with qcow2 options like refcount_bits or data_file
again.

Fixes: 14f16bf9474c860ecc127a66a86961942319f7af
       ("qemu-img: Support bitmap --merge into backing image")
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20201027164416.144115-2-mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>