virsh_attach_detach_disk: Support attach|detach disk image format

1. Using qemu_storage.QemuImg to create disk image, which can specified
the image format.
2. Add a sub-case to test qcow2 disk image attach|detach.

Signed-off-by: Yanbing Du <ydu@redhat.com>
virsh_attach_detach_disk: Add a negative case to test IDE disk hotplug

Signed-off-by: Yanbing Du <ydu@redhat.com>
virsh_attach_detach_disk: Add a case to attach logical volume

Signed-off-by: Yanbing Du <ydu@redhat.com>