iotests: Provide a function for checking the creation of huge files

iotests: Provide a function for checking the creation of huge files

Some tests create huge (but sparse) files, and to be able to run those
tests in certain limited environments (like CI containers), we have to
check for the possibility to create such files first. Thus let's introduce
a common function to check for large files, and replace the already
existing checks in the iotests 005 and 220 with this function.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Tested-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20191204154618.23560-2-thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>