disable log-throttling in xen_platform after boot

disable log-throttling in xen_platform after boot

currently log throttling in xen_platform can be disabled only if a
particular key is present on xenstore at boot time.
Obviously this requirement makes it difficult to use.
This patch fixes the issue creating a watch on xenstore on the node

/local/domain/$DOMID/log-throttling

so whenever a developer wants to disable log throttling, he just has to
write 0 to /local/domain/$DOMID/log-throttling at any moment after boot.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>