init/FreeBSD: set correct PATH for xl devd

init/FreeBSD: set correct PATH for xl devd

FreeBSD init scripts don't have /usr/local/{bin/sbin} in it's PATH, which
prevents `xl devd` from working properly since hotplug scripts require the set
of xenstore cli tools to be in PATH.

While there also fix the usage of --pidfile, which according to the xl help
doesn't use "=", and add braces around XLDEVD_PIDFILE.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
(cherry picked from commit 1d52073334d7615934fe804bc656b7aab0e92ebd)