oeqa/qemurunner: Ensure we retry after BrokenPipeError

oeqa/qemurunner: Ensure we retry after BrokenPipeError

If the BrokenPipeError occurs when writing to the serial port to wake it
up, defer the write and try again (which will happen on the 5s timeout
of the select call). Why it should return ESHUTDOWN and then work later
I'm not sure but it does appear to make it work. For now we need 'working'
QA tests whilst the issue is debugged.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>