agent: handle failures on Windows' write requests.

Windows' virtio-serial driver behavior was changed so a write
request when the host is not connected results in immediate
return with an error.

Change-Id: I28ca3c1115f6b18295a29d1f87bf3144c00759e1
Signed-off-by: Gal Hammer <ghammer@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1026474