tests/boot-serial-test: Make sure that we check the timeout regularly

If the guest continuesly writes characters to the UART, we never leave
the inner while loop and thus never check whether we've reached the
timeout value. So if we fail to find the expected string in the UART
output, the test just hangs and never finishs. Use a counter to regularly
break out of the while loop to check the timeout.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1512031988-32490-2-git-send-email-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>