Previous commits removed all capabilities from per-device property
probing for:

  pci-assign
  kvm-pci-assign
  usb-host
  scsi-generic

Remove them from the virQEMUCapsDeviceProps list and get rid of the
redundant device-list-properties QMP calls.

Note that 'pci-assign' was already useless, because the QMP version
of the device is called 'kvm-pci-assign', see libvirt commit 72574808
from 2012.
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>