iothread: add poll-grow and poll-shrink parameters

These parameters control the poll time self-tuning algorithm.  They are
optional and will default to sane values if omitted.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20161201192652.9509-14-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>