passthrough: Fix MSI/MSI-X capability structure virtualization code

passthrough: Fix MSI/MSI-X capability structure virtualization code

This patch fixes MSI/MSI-X capability structure virtualization code.

Currently, xen does not support multiple message (multiple vector).
So multiple message capable field should be emulated and fixed to 0
(single vector).

With the patch, my FC-HBA works when I assign it to guest domain where
windows 2008 runs.

In addition to this, initial values of emulated registers should be
the same with initial values defined in PCI spec. If initial values
are not defined, they should be 0. The emulated field mask and
read-only field mask are also fixed.

Signed-off-by: Yuji Shimada <shimada-yxb@necst.nec.co.jp>