tapaio check return value from read()

tapaio check return value from read()

In tools/blktap/drivers/tapaio.c there is a call to read(2) whose
return value is not checked.  The attached patch attempts to do
something vaguely sensible in cases of error.

Fully comprehensive error handling in this area would be quite tough
to introduce now but at least with this change when things go wrong
you stand a chance of getting some information about what happened.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>