kexec-xen: Use correct image type for Live Update

Unlike xen_kexec_load(), xen_kexec_unload() and xen_kexec_status()
fail to distinguish between normal kexec and Xen Live Update image
types.

Fix that by introducing a new helper function that maps internal
flags to KEXEC_TYPE_*, and using it throughout kexec-xen.c.

Signed-off-by: Raphael Ning <raphning@amazon.com>
Signed-off-by: Simon Horman <horms@verge.net.au>