AMD/IOMMU: correct handling when XT's prereq features are unavailable

AMD/IOMMU: correct handling when XT's prereq features are unavailable

We should neither cause IOMMU initialization as a whole to fail in this
case (we should still be able to bring up the system in non-x2APIC or
x2APIC physical mode), nor should the remainder of the function be
skipped (as the main part of it won't get entered a 2nd time) in such an
event. It is merely necessary for the function to indicate to the caller
(iov_supports_xt()) that setup failed as far as x2APIC is concerned.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>