multiboot: fix e801 memory map

The e801 memory sizes in the multiboot structures hard-code the available
low memory to 640.  However, the value should not include the size of the
EBDA.  Fill the value in the option ROM, getting the size of low memory
from the BIOS.

Cc: Alexander Graf <agraf@suse.de>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

pc-bios/optionrom/multiboot.S

@@ -75,6 +75,13 @@ run_multiboot:
 	shr		$4, %eax
 	mov		%ax, %fs
 
+	/* Account for the EBDA in the multiboot structure's e801
+	 * map.
+	 */
+	int		$0x12
+	cwtl
+	movl		%eax, %fs:4
+
 	/* ES = mmap_addr */
 	mov 		%fs:48, %eax
 	shr		$4, %eax