pci: Make query-pci stub consistent with the real one

pci: Make query-pci stub consistent with the real one

QMP query-pci and HMP info pci can behave differently when there are
no PCI devices.  They can report nothing, like this:

    qemu-system-aarch64 -S -M spitz -display none -monitor stdio
    QEMU 7.1.91 monitor - type 'help' for more information
    (qemu) info pci

Or they can fail, like this:

    qemu-system-microblaze -M petalogix-s3adsp1800 -display none -monitor stdio
    QEMU 7.1.91 monitor - type 'help' for more information
    (qemu) info pci
    PCI devices not supported

They fail when none of the target's machines supports PCI, i.e. when
we're using qmp_query_pci() from hw/pci/pci-stub.c.

The error is not useful, and reporting nothing makes sense, so do that
in pci-stub.c, too.

Now qmp_query_pci() can't fail anymore.  Drop the dead error handling
from hmp_info_pci().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20221201121133.3813857-5-armbru@redhat.com>