target: Reject EXTENDED_COPY when emulate_3pc is disabled

This patch rejects EXTENDED_COPY when the emulate_3pc attribute has
been explicitly disabled for the receiving device.

It also adds a similar check in target_xcopy_locate_se_dev_e4() to
ignore these devices when doing a search based upon the identifier
WWN provided by EXTENDED_COPY parameter list target descriptors.

Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>