hvmloader: Load large devices into high MMIO space as needed

hvmloader: Load large devices into high MMIO space as needed

Keep track of how much mmio space is left total, as well as the amount
of "low" MMIO space (<4GiB), and only load devices into high memory if
there is not enough low memory for the rest of the devices to fit.

Because devices are processed by size in order from large to small,
this should preferentially relocate devices with large BARs to 64-bit
space.

v3:
 - Just use mmio_total rather than introducing a new variable.
 - Port to using mem_resource directly rather than low_mmio_left

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
CC: Ian Campbell <ian.campbell@citrix.com>
CC: Stefano Stabellini <stefano.stabellini@citrix.com>
CC: Hanweidong <hanweidong@huawei.com>
CC: Keir Fraser <keir@xen.org>