xen_pt: Fix passthrough of device with ROM.

QEMU does not need and should not allocate memory for the ROM of a
passthrough PCI device. So this patch initialize the particular region
like any other PCI BAR of a passthrough device.

When a guest will access the ROM, Xen will take care of the IO, QEMU
will not be involved in it.

Xen set a limit of memory available for each guest, allocating memory
for a ROM can hit this limit.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reported-and-Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
