regression from -unstable c/s 21812:e382656e4dcc (noticed in 4.0.1)

["Jan Beulich" <JBeulich@novell.com>]

Keir,

without the use of dom0_mem= reserving sufficiently much memory,
this change causes a regression as the calculation of the Dom0
allocation doesn't account for the memory consumed by
iommu_dom0_init(). With enough memory in the system, this may
run Xen out of memory altogether, in particular preventing Dom0
from setting up its swiotlb (and pointing out lack of error handling,
as it's very unlikely for hd->platform_ops->dom0_init() to have
succeeded when afterwards there's not a single page left available).

To me it's not immediately clear what the best fix for this problem
is: Either there needs to be another hook into iommu code to
pre-calculate the memory needs, or the !strict case needs to be
handled earlier again (under [questionable] the assumption that
someone forcing strict mode will know to also pass a suitable
dom0_mem=).

Thanks, Jan