kvm tools: change the binary name kvm to lkvm in Documentation

Documentation: Change the binary name from *kvm* to *lkvm*,
since commit ad3efb18 already called the binary *lkvm* and
with a alias name *vm*.

Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>