xen: fix copyright header

Some of the code in xen-locore.S was picked from Cherry G. Mathew
amd64 Xen PV branch, but I've failed to set the proper copyright, so
do it now.

Approved by: gibbs
Remove duplicate SYSCTL_DECL(_debug_acpi) which was breaking tinderbox

MFC after:	2 weeks
X-MFC-With: r264849
Make CAM target CTL frontend respect SIM I/O size limitations.

If datamove size is bigger then SIM can handle, or it has more segments
then this code can handle -- split it into several CTIO requests.