block: Allow changing 'detect-zeroes' on reopen

block: Allow changing 'detect-zeroes' on reopen

'detect-zeroes' is one of the basic BlockdevOptions available for all
drivers, but it's not handled by bdrv_reopen_prepare(), so any attempt
to change it results in an error:

   (qemu) qemu-io virtio0 "reopen -o detect-zeroes=on"
   Cannot change the option 'detect-zeroes'

Since there's no reason why we shouldn't allow changing it and the
implementation is simple let's just do it.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>