emulators/xen-kernel411 & sysutils/xen-tools411: drop version number from port

I don't have the bandwidth to maintain multiple version of Xen, and
going forward all newer version of Xen should work on FreeBSD without
issues.

Sponsored by: Citrix Systems R&D
Reviewed by: bapt
Differential revision: https://reviews.freebsd.org/D19284