libxl: write qemu arguments into separate xenstore keys

libxl: write qemu arguments into separate xenstore keys

This allows using arguments with spaces, like -append, without
nominating any special "separator" character.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Jason Andryuk <jandryuk@gmail.com>
Write arguments in dm-argv directory instead of overloading mini-os's
dmargs string.

Make libxl__write_stub_dmargs vary behaviour based on the
is_linux_stubdom flag.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>