imgstorage: apply proper options to zfs create

To get better performance with zvol we need to use the following
options:
'-o', 'primarycache=metadata', '-o', 'volblocksize=128K',

zfs create on the vm container did not use them (zfs create on the
nas was using them though)
imgstorage: create a variable with common option for zfs create

all zfs create should use the imgstoragedaemon.zfs_create variable

I have a feeling that we will need to chage this a lot.