x86/mm: don't needlessly veto migration

x86/mm: don't needlessly veto migration

Now that xl.cfg has an option to explicitly enable IOMMU mappings for a
domain, migration may be needlessly vetoed due to the check of
is_iommu_enabled() in paging_log_dirty_enable().
There is actually no need to prevent logdirty from being enabled unless
devices are assigned to a domain.

NOTE: While in the neighbourhood, the bool_t parameter type in
      paging_log_dirty_enable() is replaced with a bool and the format
      of the comment in assign_device() is fixed.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>