Fix upload losing synchronization if root not mounted (RHBZ#576879).

Fix upload losing synchronization if root not mounted (RHBZ#576879).

Modify the generator so that it can correctly handle early
cancellation for Pathname|Device|.. parameters.  This fixes
the upload command, but consequently we need to fix the
parameters for tar_in and t?z_in commands.  This should also
mean that 'win:' can now be used as the second argument of
tar_in and t?z_in commands in guestfish, whereas previously
this wouldn't have worked.

Adds a regression test for the original problem.