scsi: Clean up duplicated error in legacy if=scsi code

Commit a818a4b changed scsi_bus_legacy_handle_cmdline() to report
errors from scsi_bus_legacy_add_drive() with error_report() in
addition to returning them.  That's inappropriate.

Two kinds of callers:

1. realize methods (devices "esp", "virtio-scsi-device" and
   "spapr-vscsi")

   The error object gets passed up the call chain until it gets
   reported again and freed.

   Example:

   $ qemu-system-arm -M virt -S -display none \
   > -drive if=scsi,id=foo,bus=1,file=tmp.qcow2 \
   > -device nec-usb-xhci -device usb-storage,drive=foo \
   > -device virtio-scsi-pci
   qemu-system-arm: -drive if=scsi,id=foo,bus=1,file=tmp.qcow2: Property 'scsi-disk.drive' can't take value 'foo', it's in use
   qemu-system-arm: -drive if=scsi,id=foo,bus=1,file=tmp.qcow2: Setting drive property failed
   qemu-system-arm: -device virtio-scsi-pci: Setting drive property failed
   qemu-system-arm: -device virtio-scsi-pci: Device initialization failed
   qemu-system-arm: -device virtio-scsi-pci: Device 'virtio-scsi-pci' could not be initialized

   The second message in this error cascade comes from
   scsi_bus_legacy_handle_cmdline().  The error object then gets
   passed up to the qdev_init() called from
   virtio_scsi_pci_init_pci(), which reports it again.

2. init methods (devices "am53c974", "dc390", "lsi53c895a",
   "lsi53c810", "megasas", "megasas-gen2")

   init methods need to report their errors with qerror_report().
   These don't.  The inappropriate error_report() papers over the bug.

   error_report() isn't the same as qerror_report() in QMP context,
   but this can't actually happen: QMP can still only hot-plug, and
   callers call scsi_bus_legacy_handle_cmdline() only on cold-plug.
   Except for sysbus_esp_realize(), but that can't be hot-plugged at
   all, as far as I can tell.

Fix the init methods and drop the inappropriate error_report() in
scsi_bus_legacy_handle_cmdline().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-Id: <1425925048-15482-2-git-send-email-armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
