hw/arm: do not free machine->fdt in arm_load_dtb()

hw/arm: do not free machine->fdt in arm_load_dtb()

At this moment, arm_load_dtb() can free machine->fdt when
binfo->dtb_filename is NULL. If there's no 'dtb_filename', 'fdt' will be
retrieved by binfo->get_dtb(). If get_dtb() returns machine->fdt, as is
the case of machvirt_dtb() from hw/arm/virt.c, fdt now has a pointer to
machine->fdt. And, in that case, the existing g_free(fdt) at the end of
arm_load_dtb() will make machine->fdt point to an invalid memory region.

Since monitor command 'dumpdtb' was introduced a couple of releases
ago, running it with any ARM machine that uses arm_load_dtb() will
crash QEMU.

Let's enable all arm_load_dtb() callers to use dumpdtb properly. Instead
of freeing 'fdt', assign it back to ms->fdt.

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-arm@nongnu.org
Fixes: bf353ad55590f ("qmp/hmp, device_tree.c: introduce dumpdtb")
Reported-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-id: 20230328165935.1512846-1-armbru@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>