KVM: selftest: Add proper helpers for x86-specific save/restore ioctls

Add helpers for the various one-off helpers used by x86's vCPU state
save/restore helpers, and convert the other open coded ioctl()s to use
existing helpers.

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