device assignment: default requires IOMMU

The default mode for device assignment is to rely on an IOMMU for
proper translations and a functioning device in the guest.  The current
logic makes this requirement advisory, and simply disables the request
for IOMMU if one is not found on the host.  This makes for a confused
user when the device assignment appears to work, but the device in the
guest is not functioning  (I've seen about a half-dozen reports with
this failure mode).

Change the logic such that the default requires the IOMMU.  Period.
If the host does not have an IOMMU, device assignment will fail.

This is a user visible change, however I think the current situation is
simply broken.

And, of course, disabling the IOMMU requirement using the old:

   -pcidevice host=[addr],dma=none

or the newer:

   -device pci-assign,host=[addr],iommu=0

will do what it always did (not require an IOMMU, and fail to work
properly).

Cc: Alexander Graf <agraf@suse.de>
Cc: Dmitri Seletski <drjoms@gmail.com>
Cc: Sheng Yang <sheng@linux.intel.com>
Signed-off-by: Chris Wright <chrisw@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

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