qemu: mdev: Use vfio-pci 'display' property only with vfio-pci mdevs

qemu: mdev: Use vfio-pci 'display' property only with vfio-pci mdevs

S390 is aware of both vfio-pci and vfio-ccw devices, so
on S390 the capability QEMU_CAPS_VFIO_PCI_DISPLAY will be
available. Add an extra check to make sure we only set the
display to off for vfio-pci mediated devices. Otherwise we
add display for vfio-ccw device and this breaks vfio-ccw
device qemu command line.

Fixes: d54e45b6e conf: Introduce new <hostdev> attribute 'display'
Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
Reviewed-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>