xen: report PV capability in sysctl and use it in toolstack

xen: report PV capability in sysctl and use it in toolstack

0e2c886ef ("xen: decouple HVM and IOMMU capabilities") provided a
truth table for what `xl info` would report. In order to make the
table work xen will need to report its PV capability.

Replace cap_directio with cap_pv in libxl IDL. It is safe to do so
because cap_directio has never been released. Revert to use
cap_hvm_directio to mark the availability of IOMMU, to save us from
providing a compatibility layer.

Don't bump sysctl version number because we've already done so.

Also provide a new virt_caps "pv", change "directio" to "pv_directio".
The truth table is now:

    pv      hvm     iommu           flags in xl info
    0       0       x               n/a
    0       1       0               hvm
    0       1       1               hvm hvm_directio
    1       0       0               pv
    1       0       1               pv pv_directio
    1       1       0               pv hvm
    1       1       1               pv hvm hvm_directio pv_directio

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>