lxd/instance/exec: Adds execWS constants for websocket numbers
lxd/instance/exec: Adds execWS constants for websocket numbers

Clearer for the reader, rather than relying on magic numbers.

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

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