ArmVirtPkg: don't build the network stack uselessly for Xen

ArmVirtPkg: don't build the network stack uselessly for Xen

"ArmVirtXen.fdf" pulls in none of the drivers from
"MdeModulePkg/Universal/Network", therefore building them in
"ArmVirtXen.dsc", via "ArmVirt.dsc.inc", is wasted work.

Move the "MdeModulePkg/Universal/Network" drivers from "ArmVirt.dsc.inc"
to "ArmVirtQemu.dsc" and "ArmVirtQemuKernel.dsc".

Place the new block between the "Bds" and "SCSI Bus and Disk Driver"
blocks, similarly to its context in "ArmVirtQemuFvMain.fdf.inc".

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>