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()
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>