Osstest/Debian: Add "clk_ignore_unused" to default command line

Osstest/Debian: Add "clk_ignore_unused" to default command line

dom0 is not aware that some clocks are actually in use (e.g. by the
hypervisor), so this stops the kernel from messing with (specifically,
disabling) those clocks. It's harmless even when not needed.

Really there ought to be some interface to communicate this from Xen
to dom0, or some other mechanism to gate things. See
http://bugs.xenproject.org/xen/bug/45

This bug is present in at least Wheezy and Jessie so apply this
workaround to those.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>