KVM: nSVM: refactor nested_svm_vmrun

Analogous to 858e25c0

 ("kvm: nVMX: Refactor nested_vmx_run()"), this commit splits
nested_svm_vmrun into two parts. The newly introduced enter_svm_guest_mode modifies the
vcpu state to transition from L1 to L2, while the code left in nested_svm_vmrun handles
the VMRUN instruction.

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