KVM: nVMX: Implement VMREAD and VMWRITE

Implement the VMREAD and VMWRITE instructions. With these instructions, L1
can read and write to the VMCS it is holding. The values are read or written
to the fields of the vmcs12 structure introduced in a previous patch.

Signed-off-by: Nadav Har'El <nyh@il.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>