libxl: Support PV consoles

libxl: Support PV consoles

Currently the driver only exposes the ability to connect to the serial console
of a Xen guest, which doesn't work for a PV guest. Since for an HVM guest the
serial devices are duplicated as consoles it is sufficient to just use the
console devices unconditionally.

Tested with the following bit of config XML:

<domain type='xen'>
  ...
  <devices>
    <console type='pty'>
      <target type='xen'/>
    </console>
  </devices>
</domain>

I have observed and tested this on ARM but I believe it also applies to x86 PV
guests.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Jim Fehlig <jfehlig@suse.com>
Cc: Dario Faggioli <dario.faggioli@citrix.com>
Cc: Clark Laughlin <clark.laughlin@linaro.org>