KVM: selftests: Sanity check input to ioctls() at build time

Add a static assert to the KVM/VM/vCPU ioctl() helpers to verify that the
size of the argument provided matches the expected size of the IOCTL.
Because ioctl() ultimately takes a "void *", it's all too easy to pass in
garbage and not detect the error until runtime.  E.g. while working on a
CPUID rework, selftests happily compiled when vcpu_set_cpuid()
unintentionally passed the cpuid() function as the parameter to ioctl()
(a local "cpuid" parameter was removed, but its use was not replaced with
"vcpu->cpuid" as intended).

Tweak a variety of benign issues that aren't compatible with the sanity
check, e.g. passing a non-pointer for ioctls().

Note, static_assert() requires a string on older versions of GCC.  Feed
it an empty string to make the compiler happy.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>