lttng-ctl: fix: possible unaligned access in packed structure

Wrap all lttng_domain copies with COPY_DOMAIN_PACKED which copies the
source domain to a temporary destination (on stack) and then assign
this temporary domain to the destination domain. This ensures the
compiler generates the code needed to perform the unaligned accesses
to the domain.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>