Modify get_disk_attr function according to PR#1780
Fix some bugs on rhel6.
Modify iscsi device creation method; delete usb tablet device before add it.
Modify operations on osxml according virt-test#1790.
Remove 0x prefix for wwn option, as some libvirt version doesn't suppport it.
Merge pull request #266 from rbian/disk-options

Modify get_disk_attr function according to PR#1780