lxd/device/disk: Add vmVirtfsProxyHelperPaths and vmVirtiofsdPaths functions
lxd/device/disk: Add vmVirtfsProxyHelperPaths and vmVirtiofsdPaths functions

And standardise the location of the virtiofsd socket path in the instance's devices directory (like its pid file).

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

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