hw/core: implement a guest-loader to support static hypervisor guests

hw/core: implement a guest-loader to support static hypervisor guests

Hypervisors, especially type-1 ones, need the firmware/bootcode to put
their initial guest somewhere in memory and pass the information to it
via platform data. The guest-loader is modelled after the generic
loader for exactly this sort of purpose:

  $QEMU $ARGS  -kernel ~/xen.git/xen/xen \
    -append "dom0_mem=1G,max:1G loglvl=all guest_loglvl=all" \
    -device guest-loader,addr=0x42000000,kernel=Image,bootargs="root=/dev/sda2 ro console=hvc0 earlyprintk=xen" \
    -device guest-loader,addr=0x47000000,initrd=rootfs.cpio

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210303173642.3805-5-alex.bennee@linaro.org>