xend: Enable to set config variables in /etc/sysconfig/xend

xend: Enable to set config variables in /etc/sysconfig/xend

The attached patch enables to set the environment variables for xend
in /etc/sysconfig/xend.

There are four variables.

XENCONSOLED_TRACE=3D[none|guest|hv|all]
XENSTORED_ROOTDIR=3D/var/lib/xenstored
XENSTORED_TRACE=3D[yes|on|1]
XENBACKENDD_DEBUG=3D[yes|on|1]

The XENCONSOLED_TRACE and XENSTORED_ROOTDIR take strings for each
command's options. And if thease variables have non-zero strings, then
export them.
If the XENSTORED_TRACE and XENBACKENDD_DEBUG take either "yes", "on"
or "1" then export them.

From: Kazuhiro SUZUKI <kaz@jp.fujitsu.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>