passthough: add no_wb option for pci conf write

passthough: add no_wb option for pci conf write

Current pt_pci_write_config always writes back to real pci conf
space. However, in the case of MSI address and data registers,
if guest changes the affinity of the interrupt, stale data will
be written to these registers. This is particularly a problem
if Xen uses per-CPU vector, where the interrupt in question fails
to work. This patch fixes this by adding an option to disable the
write back of certain controls.

Signed-off-by: Qing He <qing.he@intel.com>