KVM: be more resilient on broken migration answers

Before, when doing kvm live migrations we use to accept an "unknown
status" but to reject anything that didn't match our regexp. Since we've
seen "info migrate" return a completely empty answer, we'll be more
tolerant of completely unknown results (while still logging them) and at
the same time we'll limit the number of them which we're willing to
accept in a row.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>