xen/pciback: Disable MSI/MSI-X when reseting a device

In cases where the guest is abruptly killed and has not disabled
MSI/MSI-X interrupts we want to do it for it.

Otherwise when the guest is started up and enables MSI, we would
get a WARN() that the device already had been enabled.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>