virsh_change_media: Backup and restore original vm xml

The original xml of the vm need to be backed up before test
and restored after it to keep the xml unchanged.

Signed-off-by: Hao Liu <hliu@redhat.com>
libvirt: virsh_edit: Add maximum option for vcpucount

* In RHEL-6, vcpucount --config option must be used with --maximum
  or --active option, using only --config option with cause error.

* Fix a PEP8 (too long line).

Signed-off-by: Hao Liu <hliu@redhat.com>