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>