d/xen-utils-common.xen.init: use /run instead of /var/run

d/xen-utils-common.xen.init: use /run instead of /var/run

/var/run has been a symlink to /run for ages, and since Stretch it's
guaranteed to be available. Adapt the paths in here to use /run
directly.

Debian Policy Manual v4.0.0, Released May, 2017 checklist says:

    9.1.1 The stable release of Debian supports /run, so packages may
    now assume that it exists and do not need any special dependency on
    a version of initscripts.

Signed-off-by: Hans van Kranenburg <hans@knorrie.org>