bios: Make sure PVH still works with "rom" feature

Without this change, QEMU/CloudHV will attempt to load the ROM into the
memory region below 4GiB. But something (i.e. SeaBIOS) is already there.

We just pick an arbitrary address. It doesn't actually matter where it
gets loaded, as the ROM code isn't used when doing PVH boot.

layout.ld

@@ -4,7 +4,9 @@ PHDRS
 {
   ram  PT_LOAD FILEHDR PHDRS ;
   note PT_NOTE               ;
-  rom  PT_LOAD               ;
+  /* We set the PhysAddr to an arbitrary RAM address, so that the ROM will still
+     boot with PVH. Without this, this PHDR conflicts with QEMU's SeaBIOS.    */
+  rom  PT_LOAD AT(ram_max)   ;
 }
 
 /* Loaders like to put stuff in low memory (< 1M), so we don't use it. */