lxd: Fix failing backup import
lxd: Fix failing backup import

Instead of failing right away when no root device can be found in
devices, check expanded devices as well. Fail only if no root device
could be found.

Signed-off-by: Thomas Hipp <5f82c492b3b00e427412d216ce820707a10c51ce@canonical.com>

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