Revert "copy: Always allow multiple connections"

This reverts commit bfd7ec3e.

This doesn't work with qemu-nbd because it appears to internally
prevent multiple connections to the same export name.  Using this
patch would require using qemu-nbd without locking[1] which would be
unsafe.

[1] ie. a theoretical qemu-nbd -U option which qemu-nbd doesn't seem
to support