x86/pvh: change the order of the iommu initialization for Dom0

x86/pvh: change the order of the iommu initialization for Dom0

The iommu initialization will also create MMIO mappings in the Dom0
p2m, so the paging memory pool needs to be allocated or else iommu
initialization will fail.

Move the call to init the iommu after the Dom0 p2m has been setup in
order to solve this.

Note that issues caused by this wrong ordering have only been seen
when using shadow paging.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>