mountfsd: also return suggested mount point paths for the returned partitions

mountfsd: also return suggested mount point paths for the returned partitions

When mounting a disk image we return a bunch of mount fds referencing
the various partitions in the disk, along with some metadata about them.
One key metadata field is the "designator" which is supposed to tell
clients what is what, and where to mount it.

Let's make this more explicit: let's also include the literal relative
path where each mount shall be placed, to simplify implementations of
clients that do not care about the concept of designators.