KVM: nVMX: Refactor handle_vmwrite

Refactor existent code so we re-use vmcs12_write_any to copy fields from the
shadow vmcs specified by the link pointer (used by the processor,
implementation-specific) to the VMCS12 software format used by L0 to hold
the fields in L1 memory address space.

Signed-off-by: Abel Gordon <abelg@il.ibm.com>
Reviewed-by: Orit Wasserman <owasserm@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>