libxl: allow to set the ratelimit value online for Credit2

libxl: allow to set the ratelimit value online for Credit2

This is the remaining part of the plumbing (the libxl
one) necessary to be able to change the value of the
ratelimit_us parameter online, for Credit2 (like it is
already for Credit1).

Note that, so far, we were rejecting (for Credit1) a
new value of zero, despite it is a pretty nice way to
ask for the rate limiting to be disabled, and the
hypervisor is already capable of dealing with it in
that way.

Therefore, we change things so that it is possible to
do so, both for Credit1 and Credit2

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>