xen: Make XEN_MAX_DOMAIN_MEMORY have more sensible defaults

Which is that 128GB is not going to happen with 32-bit PV DomU.
Lets use something more realistic. Also update the 64-bit to 500GB
which is the max a PV guest can do.
Signed-off-by: Maxim Uvarov <maxim.uvarov@oracle.com>
[v1: Updated 128GB->500GB for 64-bit]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/xen/Kconfig

@@ -29,7 +29,8 @@ config XEN_PVHVM
 
 config XEN_MAX_DOMAIN_MEMORY
        int
-       default 128
+       default 500 if X86_64
+       default 64 if X86_32
        depends on XEN
        help
          This only affects the sizing of some bss arrays, the unused
@@ -48,3 +49,4 @@ config XEN_DEBUG_FS
 	help
 	  Enable statistics output and various tuning options in debugfs.
 	  Enabling this option may incur a significant performance overhead.
+