iotests: add qemu_img_map() function

iotests: add qemu_img_map() function

Add a qemu_img_map() function by analogy with qemu_img_measure(),
qemu_img_check(), and qemu_img_info() that all return JSON information.

Replace calls to qemu_img_pipe('map', '--output=json', ...) with this
new function, which provides better diagnostic information on failure.

Note: The output for iotest 211 changes, because logging JSON after it
was deserialized by Python behaves a little differently than logging the
raw JSON document string itself.
(iotests.log() sorts the keys for Python 3.6 support.)

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20220321201618.903471-11-jsnow@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>