The DX register is supposed to contain the required alignment for the
allocation. It was zero, and SeaBIOS doesn't (well, didn't) cope well
with that. Set it appropriately, and set BX to indicate the regions
it's OK to allocate in too. That was already OK but let's make sure
it's initialised properly and not just working by chance.

Also actually return an error if the allocation fails. Instead of going
all the way through into the CSM and just letting it have a bogus
pointer to the E82o data.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
---
I made SeaBIOS cope with the zero too:
https://mail.coreboot.org/hyperkitty/list/seabios@seabios.org/thread/4PHW3O3Y3HJFENODFV5INBGDLZMXA5KE/

 OvmfPkg/Csm/LegacyBiosDxe/LegacyBootSupport.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/OvmfPkg/Csm/LegacyBiosDxe/LegacyBootSupport.c
b/OvmfPkg/Csm/LegacyBiosDxe/LegacyBootSupport.c
index 211750c012..e7766eb2b1 100644
--- a/OvmfPkg/Csm/LegacyBiosDxe/LegacyBootSupport.c
+++ b/OvmfPkg/Csm/LegacyBiosDxe/LegacyBootSupport.c
@@ -928,7 +928,9 @@ GenericLegacyBoot (
   if (CopySize > Private->Legacy16Table->E820Length) {
     ZeroMem (&Regs, sizeof (EFI_IA32_REGISTER_SET));
     Regs.X.AX = Legacy16GetTableAddress;
+    Regs.X.BX = (UINT16) 0x3; // Region
     Regs.X.CX = (UINT16) CopySize;
+    Regs.X.DX = (UINT16) 0x4; // Alignment
     Private->LegacyBios.FarCall86 (
       &Private->LegacyBios,
       Private->Legacy16Table->Compatibility16CallSegment,
@@ -942,6 +944,7 @@ GenericLegacyBoot (
     Private->Legacy16Table->E820Length  = (UINT32) CopySize;
     if (Regs.X.AX != 0) {
       DEBUG ((EFI_D_ERROR, "Legacy16 E820 length insufficient\n"));
+      return EFI_OUT_OF_RESOURCES;
     } else {
       CopyMem (
         (VOID *)(UINTN) Private->Legacy16Table->E820Pointer,