qemu: migration: Split out setup of the migration target
qemu: migration: Split out setup of the migration target

Separate out allocation of the virStorageSource corresponding to the
target NBD export of the migration.

As part of the splitout we allocate the export name explicitly as that
one must not change regardless whether blockdev is used or not to
provide compatibility.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Eric Blake <a9da18479c871fe903d9590822cfe2629c6a4d11@redhat.com>

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