mount/unmount foreign folder bug

[luis4a0]

This happens when mounting a folder in the client using a trailing slash in the destination path, issuing multipass mount path/to/mount diplomatic-lungfish:mountpoint/.

When trying to unmount it, the path must have the final slash. This is, the command
multipass umount diplomatic-lungfish:mountpoint/ will work, but
multipass umount diplomatic-lungfish:mountpoint won't, the command will print
"mountpoint" is not mounted"
mountpoint" not found in database.

This also happens if mounting without using a final backslash and trying to unmount using one.

This bug is related to #1378 .

[Saviq]

I think what we can do is apply QDir::cleanPath to the input, this should give us enough uniformity.