xen: decouple HVM and IOMMU capabilities

xen: decouple HVM and IOMMU capabilities

HVM and IOMMU are two distinct hardware features, yet they were
bundled together in sysctl and xl's output.

Decouple them on sysctl level. On toolstack level we still need to
maintain a sensible semantics for `xl info`. Massage the information
according to the following table:

pv      hvm     iommu           flags in xl info
0       0       0               n/a
0       0       1               n/a
0       1       0               hvm
0       1       1               hvm hvm_directio
1       0       0               NIL
1       0       1               directio
1       1       0               hvm
1       1       1               hvm hvm_directio directio

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>