kvm: nVMX: Refactor nested_vmx_run()

Nested_vmx_run is split into two parts: the part that handles the
VMLAUNCH/VMRESUME instruction, and the part that modifies the vcpu state
to transition from VMX root mode to VMX non-root mode. The latter will
be used when restoring the checkpointed state of a vCPU that was in VMX
operation when a snapshot was taken.

Signed-off-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>