lxd/instance/exec: Remove internal PTY/TTY emulation use on LXD side for VM exec
lxd/instance/exec: Remove internal PTY/TTY emulation use on LXD side for VM exec

Instead pass-through all data to the PTY/TTY emulated in the lxd-agent.

Fixes #9542

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

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