Use qubes.StartApp service by default for qvm-run when no --pass-io specified

[rootkovska]

As discussed in #3012. While this is strictly not a bug, not fixing this for 4.0 release we can be sure to receive many bug reports for qvm-run not working by default, especially for DispVMs.

[marmarek]

Note that the argument to qubes.StartApp is not a command name, but application name, as in /usr/share/applications. While for xterm or firefox it is the same, for others may not be: for example Nautilus is org.gnome.Nautilus.
This is done to allow "staring application in a VM", without giving arbitrary command execution.
#3012 require different solution, but lets discuss is there. Not in (this) duplicate.

[marmarek]

Note that the argument to qubes.StartApp is not a command name, but application name, as in /usr/share/applications. While for xterm or firefox it is the same, for others may not be: for example Nautilus is org.gnome.Nautilus.
This is done to allow "staring application in a VM", without giving arbitrary command execution.
#3012 require different solution, but lets discuss is there. Not in (this) duplicate.