IOMMU: make page table population preemptible

IOMMU: make page table population preemptible

Since this can take an arbitrary amount of time, the rooting domctl as
well as all involved code must become aware of this requiring a
continuation.

The subject domain's rel_mem_list is being (ab)used for this, in a way
similar to and compatible with broken page offlining.

Further, operations get slightly re-ordered in assign_device(): IOMMU
page tables now get set up _before_ the first device gets assigned, at
once closing a small timing window in which the guest may already see
the device but wouldn't be able to access it.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tim Deegan <tim@xen.org>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Xiantao Zhang <xiantao.zhang@intel.com>