kvm: don't pass whole extra as a single argument

If extra is longer than a single word passing it the way it was passed
won't work. Of course this solution is also only partial, as won't take
into account any form of quoting. :/

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>