lxd/device/disk: Add DiskVirtiofsdSockMountOpt constant
lxd/device/disk: Add DiskVirtiofsdSockMountOpt constant

Used for passing the virtiofsd socket path mount option name between disk device and qemu driver packages.

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

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