KVM: nVMX: Add VMX to list of supported cpuid features

If the "nested" module option is enabled, add the "VMX" CPU feature to the
list of CPU features KVM advertises with the KVM_GET_SUPPORTED_CPUID ioctl.

Qemu uses this ioctl, and intersects KVM's list with its own list of desired
cpu features (depending on the -cpu option given to qemu) to determine the
final list of features presented to the guest.

Signed-off-by: Nadav Har'El <nyh@il.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>