kvm tool: Make kvm structure to carry name copy

If default guest name is used (which is the default
case) the kvm might end up carrying the pointer to
a name which is allocated on stack.

kvm_cmd_run_init
  (on stack) default_name
  kvm__init(..., default_name)
    kvm->name = default_name

So make it to carry a copy of name.

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>