libxl_conf: Implement hyperv domain flags for Xen

libxl_conf: Implement hyperv domain flags for Xen

Adds support for configuring <hyperv/> flags for domains
running under Xen.

The following flags, making use of QEMU's existing flags, are now
configurable for Xen: vapic, synic, stimer, frequencies, tlbflush and
ipi.

Tests have been added validating translation to libxl's viridian flags

Updated docs section on <hyperv/> flags to note support and to specify
which flags work with Xen.

Signed-off-by: Will <tcosprojects@gmail.com>
Reviewed-by: Jim Fehlig <jfehlig@suse.com>