qemuDomainAttachDiskGeneric: Implement hotplug of <transient> disk
qemuDomainAttachDiskGeneric: Implement hotplug of <transient> disk

Add code which creates the transient overlay after hotplugging the disk
backend before attaching the disk frontend.

The state of the topmost image is modified to be already read-only to
prevent the need to open the image in read-write mode.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>
Reviewed-by: Pavel Hrdina <d4772d05997b8abf035041e3b4f4996380ea7e7a@redhat.com>

nertpinx/libvirt,nertpinx/libvirt,olafhering/libvirt,libvirt/libvirt,jfehlig/libvirt,nertpinx/libvirt,zippy2/libvirt,crobinso/libvirt,olafhering/libvirt,jfehlig/libvirt,olafhering/libvirt,jfehlig/libvirt,crobinso/libvirt,zippy2/libvirt,libvirt/libvirt,crobinso/libvirt,zippy2/libvirt,olafhering/libvirt,nertpinx/libvirt,libvirt/libvirt,crobinso/libvirt,jfehlig/libvirt,zippy2/libvirt,libvirt/libvirt,nertpinx/libvirt