Change grub's root device to xen/xvda,msdos1 for the x86_xen_domu
target so that it will boot without further changes.
Signed-off-by: Wilfried Klaebe <w+lede-project@chaos.in-kiel.de>
[Jo-Philipp Wich: fixed and rebased patch from FS#264, added subject]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>