-device T,help crashes

[Markus Armbruster <armbru@redhat.com>]

Watch this:

    $ qemu-system-aarch64 -S -display none -M virt -device acpi-ged,help
    qemu-system-aarch64: ../hw/core/qdev.c:858: qdev_get_machine: Assertion `dev' failed.

A number of devices crash this way:

* acpi-ged (aarch64 arm i386 loongarch64 x86_64)
* fsl-imx6 (aarch64 arm)
* fsl-imx7 (aarch64 arm)
* fsl-imx8mp (aarch64)
* microchip.pfsoc (riscv64)
* riscv.sifive.e.soc (riscv32 riscv64)
* xlnx-zynqmp (aarch64)

Here's another one:

    $ qemu-system-sparc64 -S -display none -M virt -device sun4u-iommu,help
    qemu-system-sparc64: ../system/physmem.c:1401: register_multipage: Assertion `num_pages' failed.

Devices crashing this way:

* Loongson-3A4000-mips64-cpu (mips64 mips64el)
* dc232b-xtensa-cpu (xtensa)
* dc233c-xtensa-cpu (xtensa)
* de212-xtensa-cpu (xtensa)
* de233_fpu-xtensa-cpu (xtensa)
* dsp3400-xtensa-cpu (xtensa)
* fsf-xtensa-cpu (xtensaeb)
* lx106-xtensa-cpu (xtensa)
* sample_controller-xtensa-cpu (xtensa)
* sun4m-iommu (sparc)
* sun4u-iommu (sparc64)
* test_kc705_be-xtensa-cpu (xtensaeb)
* test_mmuhifi_c3-xtensa-cpu (xtensa)

These crashes escape tests/qtest/device-introspect-test, because it
covers only HMP "device_add T,help", not CLI "-device T,help".