libxl: check HVM direct boot parameters

libxl: check HVM direct boot parameters

We should honour -initrd and -append iff -kernel is specified, because
that's how QEMU works.

Check direct boot parameters in libxl__domain_build_info_setdefault.
Return failure if kernel is missing while ramdisk (-initrd) and / or
cmdline (-append) is present.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Chunyan Liu <cyliu@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>