hw/pci: Introduce pci_requester_id()

hw/pci: Introduce pci_requester_id()

For GICv3 ITS implementation we are going to use requester IDs in KVM IRQ
routing code. This patch introduces reusable convenient way to obtain this
ID from the device pointer. The new function is now used in some places,
where the same calculation was used.

MemTxAttrs.stream_id also renamed to requester_id in order to better
reflect semantics of the field.

Signed-off-by: Pavel Fedin <p.fedin@samsung.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <5814bcb03a297f198e796b13ed9c35059c52f89b.1444916432.git.p.fedin@samsung.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>