conf: add xen hypervisor feature 'passthrough'

conf: add xen hypervisor feature 'passthrough'

'passthrough' is Xen-Specific guest configuration option new to Xen 4.13
that enables IOMMU mappings for a guest and hence whether it supports PCI
passthrough. The default is disabled. See the xl.cfg(5) man page and
xen.git commit babde47a3fe for more details.

The default state of disabled prevents hotlugging PCI devices. However,
if the guest configuration contains a PCI passthrough device at time of
creation, libxl will automatically enable 'passthrough' and subsequent
hotplugging of PCI devices will also be possible. It is not possible to
unconditionally enable 'passthrough' since it would introduce a migration
incompatibility due to guest ABI change. Instead, introduce another Xen
hypervisor feature that can be used to enable guest PCI passthrough

  <features>
    <xen>
      <passthrough state='on'/>
    </xen>
  </features>

To allow finer control over how IOMMU maps to guest P2M table, the
passthrough element also supports a 'mode' attribute with values
restricted to snyc_pt and share_pt, similar to xl.cfg(5) 'passthrough'
setting .

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>