lxc/storage: Allow attaching custom block volumes to VMs
lxc/storage: Allow attaching custom block volumes to VMs

This allows attaching custom block volumes to VMs using `lxc storage
volume attach`. Previously, one had to use `lxc config device add` which
isn't as intuitive.

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

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