Target plugin: Fix volume_create() when pass size less than 512

 * Targetd is using LibLVM which do 'size / SECTOR_SIZE'. (SECTOR_SIZE == 512)
   This will cause two issue:
    1. When less than 512, LibLVM will raise error:
        Unable to create LV without size.
    2. New volume size is always less or equal to requested.
       But LVM definition require new volume size bigger or equal than
       requested.

 * Just do a 512 round up would workaround this issue.

Changes in V2(Thanks to Tony for suggestions)
 * Use 512 instead of 1 >> 9.
 * Round up size to 512 before passing to targetd.

Signed-off-by: Gris Ge <fge@redhat.com>