virsh: undefine: Clarify help string for --snapshots-metadata
virsh: undefine: Clarify help string for --snapshots-metadata

Reword the end of the help string to make it more obvious that the VM
must be inactive.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Fabiano Fidêncio <2931bae082ef9bc04523ba759c03a4778316d48f@redhat.com>
ACKed-by: Eric Blake <a9da18479c871fe903d9590822cfe2629c6a4d11@redhat.com>

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