virttest.utils_test.libvirt: Add export_options in pre_pool params

Add support setup netfs with export options like 'root_squash' other
than default 'no_root_squash'.

Signed-off-by: Wayne Sun <gsun@redhat.com>
utils_test.libvirt: Fix exportfs cleanup problem with netfs pool

The nfs exportfs did not clean up after run, it caused by export_dir
not specified when use cleanup_pool.

Signed-off-by: Wayne Sun <gsun@redhat.com>
Merge pull request #1812 from waynesun09/pool_vol_test

utils_test.libvirt: Update pre_pool and fix netfs exportfs cleanup