x86/pvh: fix population of the low 1MB for dom0

x86/pvh: fix population of the low 1MB for dom0

RMRRs are setup ahead of populating the p2m and hence the ASSERT when
populating the low 1MB needs to be relaxed when it finds an existing
entry: it's either RAM or a RMRR resulting from the IOMMU setup.

Rework the logic a bit and introduce a local mfn variable in order to
assert that if the gfn is populated and not RAM it is an identity map.

Fixes: 6b4f6a31ac ('x86/PVH: de-duplicate mappings for first Mb of Dom0 memory')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>