lxd/db/instances: Ensure correct pool name is returned in GetInstancePool
lxd/db/instances: Ensure correct pool name is returned in GetInstancePool

This fixes an issue when:

 - Two storage pools
 - Two projects
 - Instance called `c1` is created in first project and storage pool
 - Instance called `c1` is created in second project and storage pool

This breaks because this function returns the name of the first storage pool for the second container.

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

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