lxd/storage: Check base image is available locally
lxd/storage: Check base image is available locally

This uses a bit of a shortcut by checking the filesystem for it instead
of hitting the database.

Closes #8015

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

lxc/lxd,stgraber/lxd,atxwebs/lxd,lxc/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd,atxwebs/lxd,atxwebs/lxd,stgraber/lxd,hallyn/lxd,atxwebs/lxd,stgraber/lxd,hallyn/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,lxc/lxd