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/virsystemdtest.c

@@ -24,6 +24,7 @@
 
 # include <dbus/dbus.h>
 # include <fcntl.h>
+# include <unistd.h>
 
 # define LIBVIRT_VIRSYSTEMDPRIV_H_ALLOW
 # include "virsystemdpriv.h"