vm/adminvm: adjust exception raised by AdminVM.start()
vm/adminvm: adjust exception raised by AdminVM.start()

Behave like any other running domain - raise
qubes.exc.QubesVMNotHaltedError instead of generic
qubes.exc.QubesVMError.

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