x86/io: move the list of guest to machine IO ports out of domain_iommu

x86/io: move the list of guest to machine IO ports out of domain_iommu

There's no reason to store that list inside of the domain_iommu struct, the
forwarding of guest IO ports into machine IO ports is not tied to the presence
of an IOMMU.

Move it inside of the hvm_domain struct instead.

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