anaconda: limit dom0 maxmem to 4GB to limit its overhead on big systems

Linux kernel have some memory overhead depending on maxmem. Dom0 isn't
meant to use that much memory (most should be assigned to AppVMs), so on
big systems this will be pure waste.

QubesOS/qubes-issues#1136
Fixes QubesOS/qubes-issues#1313

anaconda/pyanaconda/bootloader.py

@@ -1455,7 +1455,7 @@ class GRUB2(GRUB):
         # boot arguments
         log.info("bootloader.py: used boot args: %s ", self.boot_args)
         defaults.write("GRUB_CMDLINE_LINUX=\"%s\"\n" % self.boot_args)
-        defaults.write("GRUB_CMDLINE_XEN_DEFAULT=\"console=none dom0_mem=min:1024M\"\n")
+        defaults.write("GRUB_CMDLINE_XEN_DEFAULT=\"console=none dom0_mem=min:1024M dom0_mem=max:4096M\"\n")
         defaults.write("GRUB_DISABLE_RECOVERY=\"true\"\n")
         defaults.write("GRUB_THEME=\"/boot/grub2/themes/system/theme.txt\"\n")
         defaults.write("GRUB_DISABLE_OS_PROBER=\"true\"\n")