iotests: Add more "_require_drivers" checks to the shell-based tests

Test 051 should be skipped if nbd is not available, and 267 should
be skipped if copy-on-read is not enabled.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
2 files changed