KVM: selftests: Add a CPUID testcase for KVM_SET_CPUID2 with runtime updates

KVM: selftests: Add a CPUID testcase for KVM_SET_CPUID2 with runtime updates

Add a CPUID testcase to verify that KVM allows KVM_SET_CPUID2 after (or in
conjunction with) runtime updates.  This is a regression test for the bug
introduced by commit 93da6af3ae56 ("KVM: x86: Defer runtime updates of
dynamic CPUID bits until CPUID emulation"), where KVM would incorrectly
reject KVM_SET_CPUID due to a not handling a pending runtime update on the
current CPUID, resulting in a false mismatch between the "old" and "new"
CPUID entries.

Link: https://lore.kernel.org/all/20251128123202.68424a95@imammedo
Link: https://patch.msgid.link/20251202015049.1167490-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>