support for parallel instances on classic breaks all snap remove in lxd

[Zygmunt Krynicki]

# On Ubuntu 20.04 with LXD
lxc launch images:debian/sid sid
test -n "$PKG_PROXY" && lxc exec sid -- sh -c "echo \"Acquire::HTTP::Proxy \\\"$PKG_PROXY\\\";\" >/etc/apt/apt.conf.d/00proxy"
lxc exec sid -- apt-get update
lxc exec sid -- apt-get install -y snapd squashfuse fuse
lxc exec sid -- snap install core
lxc exec sid -- snap install --beta open-watcom
lxc exec sid -- snap run open-watcom.owcc-linux -v
lxc exec sid -- snap refresh --channel=edge core
lxc exec sid -- snap run open-watcom.owcc-linux -v
lxc exec sid -- snap remove --purge open-watcom # this fails because umount.ext4 fails

# Debugging with findmnt shows what's going on:
lxc exec sid -- findmnt

# We only umount one "instance" of open-watcom/2 - the other one is there.

├─/snap/core/8935 squashfuse fuse.squashfuse ro,nodev,relatime,user_id=0,group_id=0,allow_other
├─/snap/core18/1705 squashfuse fuse.squashfuse ro,nodev,relatime,user_id=0,group_id=0,allow_other
├─/snap/open-watcom/2 squashfuse fuse.squashfuse ro,nodev,relatime,user_id=0,group_id=0,allow_other
├─/snap /dev/sda5[/var/snap/lxd/common/lxd/storage-pools/default/containers/sid/rootfs/snap]
│ ext4 rw,relatime,errors=remount-ro
│ ├─/snap/core/8935 squashfuse fuse.squashfuse ro,nodev,relatime,user_id=0,group_id=0,allow_other
│ ├─/snap/core18/1705 squashfuse fuse.squashfuse ro,nodev,relatime,user_id=0,group_id=0,allow_other
│ └─/snap/core/9119 squashfuse fuse.squashfuse ro,nodev,relatime,user_id=0,group_id=0,allow_other

[Zygmunt Krynicki (zyga)]

Discussion with Maciek indicates that the key is /snap MS_SHARED mode we try to establish in LXD, not instances.