[XM-TEST] Improve detection of a guest having booted to prompt.

[XM-TEST] Improve detection of a guest having booted to prompt.

Lots of xm-test tests are failing with console timeouts on some machines
at the moment.  I reproduced the problem and found it was due to some
probing activity in the boot process of the -xen kernel.  The behaviour
of the current xm-test code is to assume that the boot process has
finished after performing three one-second waits for input---when the
probing activity introduces more than three one-second delays the test
suite breaks.

This patch changes the test suite to wait for the command prompt
before attempting to submit a command.  The timeout is increased from 3
seconds to 3 minutes but the wait exits early when the prompt is found
so the test suite doesn't take any longer to run.

Signed-off-by: Harry Butterworth <butterwo@uk.ibm.com>