QMP: Add QEMU's version to the greeting message

With capability negotiation support clients will only have a chance
to check QEMU's version (ie. issue 'query-version') after the
negotiation procedure is done.

It might be useful to clients to check QEMU's version before
negotiating features, though.

To allow that, this commit adds the QEMU's version object to the
greeting message.

Not really sure this is needed, but doesn't hurt anyway.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
