qapi/block-core: update documentation of preallocation parameter

Add default and available values in the documentation block of
each block device or protocol that supports the 'preallocation'
parameter during the image creation.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190524075848.23781-2-sgarzare@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>