runner: convert xcopy to use tcmu_get_dev_max_xfer_len
runner: convert xcopy to use tcmu_get_dev_max_xfer_len

The max xfer len is now cached in the tcmu_device so use
tcmu_get_dev_max_xfer_len to get it, so we do not have to
worry about tcmu_get_attribute failures.

Signed-off-by: Mike Christie <39daeb1549a929782e38aa87c4e3da7f3844b09a@redhat.com>

agrover/tcmu-runner,open-iscsi/tcmu-runner,open-iscsi/tcmu-runner