qemu/xen: Add 64 bits big bar support on qemu Currently it is assumed PCI device BAR access < 4G memory. If there is such a device whose BAR size is larger than 4G, it must access > 4G memory address. This patch enable the 64bits big BAR support on qemu. Signed-off-by: Xudong Hao <xudong.hao@intel.com> Signed-off-by: Xiantao Zhang <xiantao.zhang@intel.com> Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>