Add the earliest stage of Xen guest initialization. This checks for the
presence of Xen and sets flags to indicate the need to initialize other
pieces (or not).
This was originally implemented by Julien Grall in 2014. This is three
portions broken off of a much larger commit.
Submitted by: Elliott Mitchell <firstname.lastname@example.org>
Original implementation: Julien Grall <email@example.com>, 2014-01-13 17:40:58