Hot plugging filesystem volume into VM with relative path causes it to be mounted under /run/lxd_agent/

[tomponline]

When hot-plugging a filesystem volume into a VM the behaviour of where that volume is mounted inside the guest when using a relative path setting differs from the behaviour of hot-plugging a filesystem volume into a container.

lxc storage volume create default foo --type=filesystem
lxc launch ubuntu:24.04 c1
lxc launch ubuntu:24.04 v1 --vm

# Hot-plugging it into a container with path=mnt causes it to be mounted at /mnt as expected.
lxc config device add c1 d0 disk source=foo pool=default path=mnt
lxc exec c1 -- mount | grep mnt
/dev/mapper/vgubuntu-root on /mnt type ext4 (rw,relatime,errors=remount-ro)

# Hot-plugging it into a VM with path=mnt causes it to be mounted at /run/lxd_agent/mnt (this is wrong, it should be /mnt).
lxc config device add v1 d0 disk source=foo pool=default path=mnt
lxc exec v1 -- mount | grep mnt
lxd_d0 on /run/lxd_agent/mnt type virtiofs (rw,relatime)

Note: This does not occur when the VM instance has the disk added before it is started, it only occurs when hot-plugging the disk.

We should ensure the agent uses the same behaviour during hot-plugging as it does at boot time.