Record history of ppcemb target in common.json

We recently removed the long deprecated "ppcemb" target.  This adds a
comment in common.json about the SysEmuTarget type, recording when it was
removed.
Suggested-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

qapi/common.json

@@ -140,6 +140,8 @@
 #        prefix to produce the corresponding QEMU executable name. This
 #        is true even for "qemu-system-x86_64".
 #
+# ppcemb: dropped in 3.1
+#
 # Since: 3.0
 ##
 { 'enum' : 'SysEmuTarget',