passthrough: MSI-INTx interrupt translation support

passthrough: MSI-INTx interrupt translation support

This patch enables Xen to use MSI for MSI-capable devices as the
underlying interrupt source even if the guest does not explicitly
use it. The guest will still see an IO-APIC based INTx interrupt
translated by Xen from the MSI irq.

If the guest enables MSI or MSI-X for the passthrough device, this
translation is automatically turned off. It can also be disabled in
the config file at domain startup time.

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