virt: add missing #includes

src/basic/virt.c: In function 'detect_vm_device_tree':
src/basic/virt.c:117:17: error: unknown type name '_cleanup_closedir_'
                 _cleanup_closedir_ DIR *dir = NULL;

src/basic/virt.c:128:17: error: implicit declaration of function 'FOREACH_DIRENT' [-Werror=implicit-function-declaration]
                 FOREACH_DIRENT(dent, dir, return -errno)