target/i386: skip KVM_GET/SET_NESTED_STATE if VMX disabled, or for SVM

target/i386: skip KVM_GET/SET_NESTED_STATE if VMX disabled, or for SVM

Do not allocate env->nested_state unless we later need to migrate the
nested virtualization state.

With this change, nested_state_needed() will return false if the
VMX flag is not included in the virtual machine.  KVM_GET/SET_NESTED_STATE
is also disabled for SVM which is safer (we know that at least the NPT
root and paging mode have to be saved/loaded), and thus the corresponding
subsection can go away as well.

Inspired by a patch from Liran Alon.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>