More reliable connection with nested qemu image.

More reliable connection with nested qemu image.

Under some circumstances on a busy server, the qga channel
between the host and hte virtual guest has some garbage
characters that cause the communications to fail.
This has been seen on Centos 7.4 on a slow or busy server.

This change sends a flush to qemu guest via the qga socket at 5
second intervals until there is a non empty response from guest
before sending a guest ping.

Change-Id: I6f21c205b289169aee9d6a4072ad4e6bafafa76f
Signed-off-by: Thomas F Herbert <therbert@redhat.com>