lxd/instances: Limit shutdown concurrency to number of instances or number of CPU cores (which ever is less)
lxd/instances: Limit shutdown concurrency to number of instances or number of CPU cores (which ever is less)

Otherwise if you have hundreds of instances running, shutting down LXD can consume all system resources
and hang.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

lxc/lxd,lxc/lxd,lxc/lxd,lxc/lxd,lxc/lxd,lxc/lxd,lxc/lxd