qubes/vm/dispvm: don't crash when DispVM is already killed
qubes/vm/dispvm: don't crash when DispVM is already killed

This is regression of QubesOS/qubes-issues#1660

Fixes QubesOS/qubes-issues#1660

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