Xen: Add support for qemu command-line passthrough

Xen: Add support for qemu command-line passthrough

Xen supports passing arbitrary arguments to the QEMU device model via
the 'extra' member of the public libxl_domain_build_info structure.
This patch adds a 'xen' namespace extension, similar to the QEMU and
bhyve drivers, to map arbitrary arguments to the 'extra' member. Only
passthrough of arguments is supported. Passthrough of environment
variables or capabilities adjustments is not supported.

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