Fix the check for non-existence of the save/restore directories, by using &&

Fix the check for non-existence of the save/restore directories, by using &&
rather than -a.  The former shortcuts at the script level, whereas the latter
does not, which means that the backtick-enclosed arguments are evaluated
regardless of the success of preceding tests.

Tidy the aforementioned test into a function of its own.

Add an implementation of usleep for those systems that only have sleep (Debian,
for example).

Signed-off-by: Ewan Mellor <ewan@xensource.com>