docs: migration: remove xenmigr
docs: migration: remove xenmigr

xenmigr was for xend which was removed in in 4.3.0.

Fixes: 1dac5fbbbb06a0341e8087dc33af75c8352d77a4
Signed-off-by: Olaf Hering <ded022db89399a060a39da31922f173c6556c2d9@aepfle.de>
Reviewed-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

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