block: Fix compilation with CentOS 6

block: Fix compilation with CentOS 6

GCC 4.4 from CentOS 6 chokes in the following way:

  src/hw/blockcmd.c: In function 'scsi_rep_luns_scan':
  src/hw/blockcmd.c:229: error: unknown field 'cdbcmd' specified in initializer
  src/hw/blockcmd.c:229: warning: missing braces around initializer
  src/hw/blockcmd.c:229: warning: (near initialization for 'op.<anonymous>')
  src/hw/blockcmd.c:229: warning: initialization makes integer from pointer without a cast

Avoid using an initialiser list.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
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Discovered while adding CentOS 6 to Xen's CI system.  If this patch is
acceptable, what are the chances of having it backported to 1.12-stable ?