Add registry overrides for features

Add registry overrides for features

Add a REG_DWORD to XenVbd's Parameters key, with names based
on the features, as per xenstore value, and set to 0 to
override use of that feature.

For example,
 "HKLM\System\CCS\Services\XenVbd\Parameters" "feature-barrier" 0
  will prevent the frontend issuing BLKIF_OP_WRITE_BARRIERs
 "HKLM\System\CCS\Services\XenVbd\Parameters" "feature-discard" 0
  will prevent the frontend issuing BLKIF_OP_DISCARDs

Signed-off-by: Owen Smith <owen.smith@citrix.com>