virsh_attach_detach_disk: Add a case to cover ejcet CDROM after attach cdrom

Signed-off-by: Yanbing Du <ydu@redhat.com>
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>
virsh_attach_device: Add a negative to test invalid disk type

1. Attach-device will fail if disk type is invalid.
2. For this test, virsh cmd will fail but stderr doesn't have
'XML error' key words, and the XML content is really import for
this command, so always output it.

Signed-off-by: Yanbing Du <ydu@redhat.com>
virsh_attach_detach_disk: Restart libvirtd after attach disk

Signed-off-by: Yanbing Du <ydu@redhat.com>
virsh_attach_detach_disk: Using exist method to create image

Signed-off-by: Yanbing Du <ydu@redhat.com>
Merge pull request #193 from will-Do/at_dt_disk

virtual_disk: cases expand to cover test corners