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".