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

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.

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>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ijc -- ran autogen.sh as requested]