KVM: Add missing MODULE_DESCRIPTION()

KVM: Add missing MODULE_DESCRIPTION()

Add a module description for kvm.ko to fix a 'make W=1' warning:

  WARNING: modpost: missing MODULE_DESCRIPTION() in arch/x86/kvm/kvm.o

Opportunistically update kvm_main.c's comically stale file comment to
match the module description.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Link: https://lore.kernel.org/r/20240622-md-kvm-v2-1-29a60f7c48b1@quicinc.com
[sean: split x86 changes to a separate commit, remove stale VT-x comment]
Signed-off-by: Sean Christopherson <seanjc@google.com>