IOMMU: add phantom function support

IOMMU: add phantom function support

Apart from generating device context entries for the base function,
all phantom functions also need context entries to be generated for
them.

In order to distinguish different use cases, a variant of
pci_get_pdev() is being introduced that, even when passed a phantom
function number, would return the underlying actual device.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: "Zhang, Xiantao" <xiantao.zhang@intel.com>