Update to r52081:

The emulators/xen package does not exist anymore and there are two versions
now. One is for Xen 4.7, intended for FreeBSD 11. The other is for -CURRENT
and uses a more modern Xen 4.11. Adjust the description and examples to
differentiate the two.
The old 4.7 packages use dom0pvh=1 on the commandline, while Xen 4.11 is
using dom0=pvh. Adjust those two as well for the supported FreeBSD versions.