commit 606af829 ("KVM: selftests: arm64: Test vCPU-scoped feature ID
registers") intended to test that MPIDR_EL1 is unchanged across vCPU
reset but failed at actually doing so.

Add the missing assertion.

Link: https://lore.kernel.org/r/20240621225045.2472090-1-oliver.upton@linux.devSigned-off-by: Oliver Upton <oliver.upton@linux.dev>