q35: gigabyte alignment for ram

Map 2G (q35) of memory below 4G, so the RAM pieces
are nicely aligned to gigabyte borders.

Keep old memory layout for (a) old machine types and (b) in case all
memory fits below 4G and thus we don't have to split RAM into pieces
in the first place.  The later makes sure this change doesn't take
away memory from 32bit guests.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/i386/pc_q35.c

@@ -51,6 +51,7 @@
 static bool has_pci_info;
 static bool has_acpi_build = true;
 static bool smbios_type1_defaults = true;
+static bool gigabyte_align = true;
 
 /* PC hardware initialisation */
 static void pc_q35_init(QEMUMachineInitArgs *args)
@@ -93,8 +94,9 @@ static void pc_q35_init(QEMUMachineInitArgs *args)
     kvmclock_create();
 
     if (args->ram_size >= 0xb0000000) {
-        above_4g_mem_size = args->ram_size - 0xb0000000;
-        below_4g_mem_size = 0xb0000000;
+        ram_addr_t lowmem = gigabyte_align ? 0x80000000 : 0xb0000000;
+        above_4g_mem_size = args->ram_size - lowmem;
+        below_4g_mem_size = lowmem;
     } else {
         above_4g_mem_size = 0;
         below_4g_mem_size = args->ram_size;
@@ -228,6 +230,7 @@ static void pc_q35_init(QEMUMachineInitArgs *args)
 static void pc_compat_1_7(QEMUMachineInitArgs *args)
 {
     smbios_type1_defaults = false;
+    gigabyte_align = false;
 }
 
 static void pc_compat_1_6(QEMUMachineInitArgs *args)