storage: Serialize storage pool creation
storage: Serialize storage pool creation

Closes #4150

Signed-off-by: Stéphane Graber <089afc6d81f66f1168a9849e15660feae286e024@ubuntu.com>

atxwebs/lxd,erikmack/lxd,stgraber/lxd,atxwebs/lxd,erikmack/lxd,hallyn/lxd,stgraber/lxd,jsavikko/lxd,erikmack/lxd,stgraber/lxd,Skarlso/lxd,jsavikko/lxd,pcdummy/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,jsavikko/lxd,lxc/lxd,lxc/lxd,lxc/lxd,mjeanson/lxd,lxc/lxd,hallyn/lxd,mjeanson/lxd,stgraber/lxd,pcdummy/lxd,lxc/lxd,lxc/lxd,pcdummy/lxd,Skarlso/lxd,atxwebs/lxd,hallyn/lxd,stgraber/lxd,hallyn/lxd,Skarlso/lxd,atxwebs/lxd,mjeanson/lxd,hallyn/lxd,Skarlso/lxd