docs: Add warning about using a cleared image with VIR_MIGRATE_PARAM_MIGRATE_DISKS_DE...

docs: Add warning about using a cleared image with VIR_MIGRATE_PARAM_MIGRATE_DISKS_DETECT_ZEROES_ZEROES

The migration parameter causes zero detection to be enabled and zero
blocks are *not* transferred to the destination. This means that users
must provide pre-cleared images that read all zero, otherwise the
non-zero blocks on destination which reside in places where the source
has zero blocks would be kept intact corrupting the image.

As not transferring and overwriting the zero blocks is what the feature
is supposed to do the users need to provide the proper environment.

Document the requirement, both in API and in the virsh man page for the
'--migrate-disks-detect-zeroes' option.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>