tools: fix variable scope in in check_guests_shutdown

tools: fix variable scope in in check_guests_shutdown

libvirt-guests.sh when run with more active guests than requested to
shut down in parallel will run until it times out only shutting down
the first set of guests.

This patch fixes parallel shutdown by fixing a variable scope issue
where check_guests_shutdown unintentionally reset $guests which
prevented further progress.

Fixes: https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1688508
Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>