ivshmem: use symbolic constant for PCI ID, add to pci-ids.txt

Due to disagreement on a name that is generic enough for hw/pci/pci.h,
the symbolic constants are placed in the .c files.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
diff --git a/docs/specs/pci-ids.txt b/docs/specs/pci-ids.txt
index 2d5329a..e76b196 100644
--- a/docs/specs/pci-ids.txt
+++ b/docs/specs/pci-ids.txt
@@ -30,4 +30,6 @@
 1af4:1100  Used as PCI Subsystem ID for existing hardware devices emulated
            by qemu.
 
+1af4:1110  ivshmem device (shared memory, docs/specs/ivshmem_device_spec.txt)
+
 All other device IDs are reserved.