ArmVirtPkg/ArmVirtQemuKernel: increase slack space for DTB

The relocatable build of ArmVirtQemuKernel is designed to be executed
from RAM, and contains some scratch memory at the start of the image
to use as a stack very early on, and to preserve the DTB image received
from QEMU while it discovers and initializes memory.

It turns out that 8 KB is a bit on the small side here, especially when
executing with secure world emulation enabled, in which case there are
additional nodes present.

So increase the slack space to 32 KB.

While at it, remove a stale Xen reference that was copy/pasted when this
file was created.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Laszlo Ersek <lersek@redhat.com>