i386: load kernel on xen using DMA

i386: load kernel on xen using DMA

Kernel on Xen is loaded via fw_cfg. Previously it used non-DMA version,
which loaded the kernel (and initramfs) byte by byte. Change this
to DMA, to load in bigger chunks.
This change alone reduces load time of a (big) kernel+initramfs from
~10s down to below 1s.

This change was suggested initially here:
https://lore.kernel.org/xen-devel/20180216204031.000052e9@gmail.com/
Apparently this alone is already enough to get massive speedup.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20210426034709.595432-1-marmarek@invisiblethingslab.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>