qemu: backup: Use VIR_ERR_CHECKPOINT_INCONSISTENT when starting a backup
qemu: backup: Use VIR_ERR_CHECKPOINT_INCONSISTENT when starting a backup

If we don't have a consistent chain of bitmaps for the backup to proceed
we'd report VIR_ERR_INVALID_ARG error code, which makes it hard to
decide whether an incremental backup makes even sense.

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

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