qapi-schema: fix QEMU 1.8 references

We are moving boldly on to QEMU 2.0 in the next release.  Some patches
written at a time where we assumed 1.8 would be the next version number
managed to sneak in.

s/1.8/2.0/ in qapi-schema.json
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

qapi-schema.json

@@ -3022,7 +3022,7 @@
 #
 # @devname: #optional path of the netmap device (default: '/dev/netmap').
 #
-# Since 1.8
+# Since 2.0
 ##
 { 'type': 'NetdevNetmapOptions',
   'data': {