qubespolicy/graph: let --target option filter on actual call target
qubespolicy/graph: let --target option filter on actual call target

Not on what VM can ask for (which may be later overriden by target=
option).

Fixes QubesOS/qubes-issues#3006

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