utils_test.libvirt: Fix setup_or_cleanup_nfs()

1) Fix hard-coded export options.
2) Changed `mount_dir` to make it able to be used on absolute path.
3) Added `mount_options` keyword argument.
4) Modify define_pool() and class PoolVolumeTest to be able to recover
   SElinux status.
5) When using PoolVolumeTest and pool_type is scsi, remove scsi XML
   file when cleaning up.

Signed-off-by: Hao Liu <hliu@redhat.com>
Merge pull request #1751 from Hao-Liu/fix_setup_nfs

utils_test.libvirt: Fix setup_or_cleanup_nfs()