block: remove has_variable_length from filters

Filters automatically get has_variable_length from their underlying
BlockDriverState.  There is no need to mark them as variable-length
in the BlockDriver.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20230407153303.391121-3-pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/copy-on-read.c

@@ -259,7 +259,6 @@ static BlockDriver bdrv_copy_on_read = {
     .bdrv_co_eject                      = cor_co_eject,
     .bdrv_co_lock_medium                = cor_co_lock_medium,
 
-    .has_variable_length                = true,
     .is_filter                          = true,
 };
 

block/filter-compress.c

@@ -146,7 +146,6 @@ static BlockDriver bdrv_compress = {
     .bdrv_co_eject                      = compress_co_eject,
     .bdrv_co_lock_medium                = compress_co_lock_medium,
 
-    .has_variable_length                = true,
     .is_filter                          = true,
 };
 

block/preallocate.c

@@ -558,7 +558,6 @@ BlockDriver bdrv_preallocate_filter = {
     .bdrv_set_perm = preallocate_set_perm,
     .bdrv_child_perm = preallocate_child_perm,
 
-    .has_variable_length = true,
     .is_filter = true,
 };
 

block/replication.c

@@ -762,7 +762,6 @@ static BlockDriver bdrv_replication = {
 
     .is_filter                  = true,
 
-    .has_variable_length        = true,
     .strong_runtime_opts        = replication_strong_runtime_opts,
 };