kexec: Introduce --load-live-update for xen

Support loading a live update image for xen from kexec userspace.
For a multiboot2 Elf on a xen setup, this will:
- load the Elf into KEXEC_RANGE_MA_XEN
- load purgatory and modules into KEXEC_RANGE_MA_LIVEUPDATE
- append the Elf cmdline with " liveupdate=<size>@<addr>

v2: define xen related symbols outside of HAVE_LIBXENCTRL

Signed-off-by: Varad Gautam <vrd@amazon.de>
Signed-off-by: Simon Horman <horms@verge.net.au>