KVM: selftests: Fix typos and stale comments in kvm_util

KVM: selftests: Fix typos and stale comments in kvm_util

Fix minor documentation errors in `kvm_util.h` and `kvm_util.c`.

- Correct the argument description for `vcpu_args_set` in `kvm_util.h`,
  which incorrectly listed `vm` instead of `vcpu`.
- Fix a typo in the comment for `kvm_selftest_arch_init` ("exeucting" ->
  "executing").
- Correct the return value description for `vm_vaddr_unused_gap` in
  `kvm_util.c` to match the implementation, which returns an address "at
  or above" `vaddr_min`, not "at or below".

No functional change intended.

Reviewed-by: Andrew Jones <andrew.jones@linux.dev>
Signed-off-by: Fuad Tabba <tabba@google.com>
Link: https://patch.msgid.link/20260109082218.3236580-6-tabba@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>