Update pKVM defconfig after merging 2021.08

Two changes needed:
* rootfs name is now obsolete, and
* we need to override kernel header version (defaults to 5.13)
  because glibc will refuse to run against an older kernel.

Test: make buildroot; copy rootfs.ext4; make test
Change-Id: I12b3b5eb8f37253912d505cf19c4352f694493f1
1 file changed