pci-testdev.c: Add membar-backed option for backing membar

pci-testdev.c: Add membar-backed option for backing membar

The pci-testdev device allows for an optional BAR. We have
historically used this without backing to test that systems and OSes
can accomodate large PCI BARs. However to help test p2pdma operations
it is helpful to add an option to back this BAR with host memory.

We add a membar-backed boolean parameter and when set to true or on we
do a host RAM backing. The default is false which ensures backward
compatability.

Signed-off-by: Stephen Bates <sbates@raithlin.com>
Message-Id: <Z_6JhDtn5PlaDgB_@MKMSTEBATES01.amd.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>