Fix xenstore client which would break when the process forked

The xenstore client set up a server thread that reads responses
from the xenstore socket. This was getting terminated by the
act of forking, so no requests ever completed.

This change deferrs the creation of the thread until after the
daemonize call.

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
Merge branch 'master' of github.com:xen-org/squeezed