KVM: x86: Advertise SRSO_USER_KERNEL_NO to userspace

SRSO_USER_KERNEL_NO denotes whether the CPU is affected by SRSO across
user/kernel boundaries. Advertise it to guest userspace.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Link: https://lore.kernel.org/r/20241202120416.6054-3-bp@kernel.org