machinectl: add bind-volume / unbind-volume verbs

machinectl bind-volume MACHINE PROVIDER:VOLUME[:CONFIG][:K=V,...]
  machinectl unbind-volume MACHINE PROVIDER:VOLUME

For bind-volume, machinectl parses the SPEC with the shared
bind_volume_parse(), Acquires the storage volume from the named
provider on the machinectl side, locates the target machine's
io.systemd.MachineInstance control socket via
machine_get_control_address(), pushes the fd across, and calls
io.systemd.MachineInstance.AddStorage with name='<provider>:<volume>'
and the user-supplied config string.

For unbind-volume, machinectl just forwards the name string to
io.systemd.MachineInstance.RemoveStorage.

Volumes attached at machine startup (e.g. via systemd-vmspawn's
--bind-volume=) are rejected with StorageImmutable when the user
attempts to unbind them at runtime.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

shell-completion/bash/machinectl

@@ -48,7 +48,7 @@ _machinectl() {
         [MACHINES]='status show start stop login shell enable disable poweroff reboot pause resume terminate kill
                     image-status show-image remove export-tar export-raw'
         [MACHINES_OR_FILES]='edit cat'
-        [MACHINE_ONLY]='clone rename set-limit'
+        [MACHINE_ONLY]='clone rename set-limit bind-volume unbind-volume'
         [READONLY]='read-only'
         [FILE]='import-tar import-raw'
         [MACHINES_AND_FILES]='copy-to copy-from bind'