qrexec policy: use special symbols for 'anyvm' and 'dispvm'

[joanna]

Interpreting the _common_ words 'anyvm' and 'dispvm' in a special way (which is what is being done now) can only get the user into troubles. We should use special symbols (not normally allowed to occur in the VM name) to specify those special cases, such as e.g. '*' instead of 'anyvm', and perhaps 'disp*' (which would be consistent with DispVM nameing convention, assuming qubes.py would not allow the user to create other vms with such names)

[joanna]

Or perhaps: $anyvm, and $dispvm?

Also, make sure that the user cannot create a VM with the dame dispXX.

[rafal]

Implemented in
​http://git.qubes-os.org/?p=rafal/core.git;a=commit;h=c23cc480b8cbc6877fdc0973d3213e464d6b4ac4
prebeta2 branch.