xen/iommu: fix dev assignment on ARM after 91d4eca7

xen/iommu: fix dev assignment on ARM after 91d4eca7

Fix device assignment on ARM after 91d4eca7 "mm / iommu: split
need_iommu() into has_iommu_pt() and need_iommu_pt_sync()".

arch_iommu_populate_page_table returns -ENOSYS which causes
iommu_construct to return early, although it is not an error.

hd->status needs to be set to IOMMU_STATUS_initializing before calling
iommu_use_hap_pt, otherwise iommu_use_hap_pt will return the wrong
value.

Signed-off-by: Stefano Stabellini <stefanos@xilinx.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>