iotests/221: Discard image before qemu-img map

iotests/221: Discard image before qemu-img map

See the new comment for why this should be done.

I do not have a reproducer on master, but when using FUSE block exports,
this test breaks depending on the underlying filesystem (for me, it
works on tmpfs, but fails on xfs, because the block allocated by
file-posix has 16 kB there instead of 4 kB).

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20201207152245.66987-1-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>