qubespolicy: unify calling qrexec service
qubespolicy: unify calling qrexec service

Rely on qrexec-client resolving QUBESRPC keyword, same as in case of VM
call. This will allow applying special treatment to such calls, like
calling qubes-rpc-multiplexer directly (avoiding shell), because we have
defined protocol what can be used here.

marmarek/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin