qemu: Add -blockdev support for external snapshots
qemu: Add -blockdev support for external snapshots

Use the code for creating or attaching new storage source in the
snapshot code and switch to 'blockdev-snapshot' for creating the
snapshot itself.

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

andreabolognani/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,jardasgit/libvirt,andreabolognani/libvirt,crobinso/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,crobinso/libvirt,crobinso/libvirt,zippy2/libvirt,zippy2/libvirt,andreabolognani/libvirt,olafhering/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,crobinso/libvirt,libvirt/libvirt,jardasgit/libvirt,jfehlig/libvirt,olafhering/libvirt,libvirt/libvirt,jfehlig/libvirt,nertpinx/libvirt,nertpinx/libvirt,nertpinx/libvirt,zippy2/libvirt,libvirt/libvirt,fabianfreyer/libvirt,olafhering/libvirt,jfehlig/libvirt,nertpinx/libvirt,libvirt/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,andreabolognani/libvirt,zippy2/libvirt,jardasgit/libvirt,olafhering/libvirt