configs: Use MmioVirtIO for disk image in baremetal.py

configs: Use MmioVirtIO for disk image in baremetal.py

The baremetal platform is the platform we use for running
user supplied binaries on baremetal hardware.
(simply put, it runs provided binaries without adding
a gem5 bootloader)

Some layers of this software stack might not have a pci driver.
This might be the case for firmware images like edkII
which needs to use a block device to extract the bootloader
and/or the kernel image. Those can use the memory mapped
(in host domain) virtio block device which is already
part of the VExpress_GEM5 platforms

Change-Id: I9c6ba7e1b4566a3999fd9ba20a2bebe191dc3ef8
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39995
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>