Fix configure --with-systemd-modules-load=DIR

Currently --with-systemd=DIR1 --with-systemd-modules-load=DIR2 will set
both XEN_SYSTEMD_DIR and XEN_SYSTEMD_MODULES_LOAD to DIR1. The reason is
that both AC_ARG_WITH() use 'systemd' as the name for the internal
variable. As a result the value of the first option is also use as the
value for the second option. Fix this by using another variable name.

Please rerun autogen.sh after applying this patch.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
tools/hotplug: use configure --sysconfdir result

... instead of hardcoding values and guess where they config files may
be. Also use the result of --with-sysconfig-leaf-dir.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>