vmspawn: implement io.systemd.MachineInstance.AddStorage / RemoveStorage

vmspawn: implement io.systemd.MachineInstance.AddStorage / RemoveStorage

Wire up the runtime hotplug Varlink methods on the per-VM control
socket:

  AddStorage  → take fd from the link, look up the DiskType from the
                'config' field, build a DriveInfo flagged
                QMP_DRIVE_REMOVABLE, dispatch to
                vmspawn_qmp_add_block_device(). Reply delivered async
                by on_add_device_add_complete() once the guest sees
                the device.

  RemoveStorage → forward the user-visible name to
                  vmspawn_qmp_remove_block_device(); the existing
                  device_del / DEVICE_DELETED / blockdev-del chain
                  replies on the link.

Add SD_VARLINK_SERVER_ALLOW_FD_PASSING_INPUT to the server flags so
clients can push storage fds across via sd_varlink_push_fd().

Maps -EEXIST → StorageExists and -EOPNOTSUPP/-EINVAL →
ConfigNotSupported in the AddStorage handler so callers see the
specific MachineInstance errors.

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