Support R4.0 method of xen-efi boot

Restore the R4.0 method of booting Xen on EFI. This occurs alongside the
new grub-efi boot mechanism.

kernel.spec.in

@@ -514,6 +514,12 @@ if [ -f /etc/default/grub ]; then
     fi
 fi
 
+if [ -f /boot/efi/EFI/qubes/xen.cfg ]; then
+    if ! grep -q plymouth.ignore-serial-consoles /boot/efi/EFI/qubes/xen.cfg; then
+        sed -i 's/kernel=.*/& plymouth.ignore-serial-consoles/g' /boot/efi/EFI/qubes/xen.cfg
+    fi
+fi
+
 /bin/kernel-install add %{kernelrelease} /boot/vmlinuz-%{kernelrelease} || exit $?
 
 # Generate legacy boot grub config