Fix pool related cases to recover selinux state.

This PR depends on https://github.com/autotest/virt-test/pull/1751.
This commit is base on a virt-test change to recover selinux state
after creating an NFS pool. Besides this change, this commit also
fixed a few bugs.
1) Stop virsh.domblkerror cleanup entire temp directory.
2) Clean virsh.vol_create_from and virsh.vol_download_upload file.
3) Create the file before chmod in virsh.vol_download_upload to
   prevent error.

Signed-off-by: Hao Liu <hliu@redhat.com>
`write_file()` was called upon `vol_path` but not `file_path`. Is it a typo?