iotests: replace unchecked calls to qemu_img_pipe()

iotests: replace unchecked calls to qemu_img_pipe()

qemu_img_pipe() discards the return code from qemu-img in favor of
returning just its output. Some tests using this function don't save,
log, or check the output either, though, which is unsafe.

Replace all of these calls with a checked version.

Tests affected are 194, 202, 203, 234, 262, and 303.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220321201618.903471-13-jsnow@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>