Include unistd.h where used

Include unistd.h in all files that use it, instead
of relying on it being pulled in via virutil.h

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>

tests/virfiletest.c

@@ -19,6 +19,7 @@
 #include <config.h>
 
 #include <fcntl.h>
+#include <unistd.h>
 
 #include "testutils.h"
 #include "virfile.h"