qemu-xen-trad: fix msi_translate with PV event delivery

qemu-xen-trad: fix msi_translate with PV event delivery

When switching from msitranslate to straight msi we need to make sure
that we respect PV event delivery for the msi if the guest asked for it:

- completely disable MSI on the device in pt_disable_msi_translate;
- then enable MSI again (pt_msi_setup), mapping the correct pirq to it.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Tested-by: Rolu <rolu@roce.org>