spapr: Use address from elf parser for kernel address

tl;dr: This allows Big Endian zImage booting via -kernel + x-vof=on.

QEMU loads the kernel at 0x400000 by default which works most of
the time as Linux kernels are relocatable, 64bit and compiled with "-pie"
(position independent code). This works for a little endian zImage too.

However a big endian zImage is compiled without -pie, is 32bit, linked to
0x4000000 so current QEMU ends up loading it at
0x4400000 but keeps spapr->kernel_addr unchanged so booting fails.

This uses the kernel address returned from load_elf().
If the default kernel_addr is used, there is no change in behavior (as
translate_kernel_address() takes care of this), which is:
LE/BE vmlinux and LE zImage boot, BE zImage does not.
If the VM created with "-machine kernel-addr=0,x-vof=on", then QEMU
prints a warning and BE zImage boots.

Note #1: SLOF (x-vof=off) still cannot boot a big endian zImage as
SLOF enables MSR_SF for everything loaded by QEMU and this leads to early
crash of 32bit zImage.

Note #2: BE/LE vmlinux images set MSR_SF in early boot so these just work;
a LE zImage restores MSR_SF after every CI call and we are lucky enough
not to crash before the first CI call.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Tested-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Fabiano Rosas <farosas@linux.ibm.com>
Message-Id: <20220504065536.3534488-1-aik@ozlabs.ru>
[danielhb: use PRIx64 instead of lx in warn_report]
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>

hw/ppc/spapr.c

@@ -2975,14 +2975,16 @@ static void spapr_machine_init(MachineState *machine)
     }
 
     if (kernel_filename) {
+        uint64_t loaded_addr = 0;
+
         spapr->kernel_size = load_elf(kernel_filename, NULL,
                                       translate_kernel_address, spapr,
-                                      NULL, NULL, NULL, NULL, 1,
+                                      NULL, &loaded_addr, NULL, NULL, 1,
                                       PPC_ELF_MACHINE, 0, 0);
         if (spapr->kernel_size == ELF_LOAD_WRONG_ENDIAN) {
             spapr->kernel_size = load_elf(kernel_filename, NULL,
                                           translate_kernel_address, spapr,
-                                          NULL, NULL, NULL, NULL, 0,
+                                          NULL, &loaded_addr, NULL, NULL, 0,
                                           PPC_ELF_MACHINE, 0, 0);
             spapr->kernel_le = spapr->kernel_size > 0;
         }
@@ -2992,6 +2994,13 @@ static void spapr_machine_init(MachineState *machine)
             exit(1);
         }
 
+        if (spapr->kernel_addr != loaded_addr) {
+            warn_report("spapr: kernel_addr changed from 0x%"PRIx64
+                        " to 0x%"PRIx64,
+                        spapr->kernel_addr, loaded_addr);
+            spapr->kernel_addr = loaded_addr;
+        }
+
         /* load initrd */
         if (initrd_filename) {
             /* Try to locate the initrd in the gap between the kernel