KVM nVMX: Check Host Segment Registers and Descriptor Tables on vmentry of nested guests

According to section "Checks on Host Segment and Descriptor-Table
Registers" in Intel SDM vol 3C, the following checks are performed on
vmentry of nested guests:

   - In the selector field for each of CS, SS, DS, ES, FS, GS and TR, the
     RPL (bits 1:0) and the TI flag (bit 2) must be 0.
   - The selector fields for CS and TR cannot be 0000H.
   - The selector field for SS cannot be 0000H if the "host address-space
     size" VM-exit control is 0.
   - On processors that support Intel 64 architecture, the base-address
     fields for FS, GS and TR must contain canonical addresses.

Signed-off-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
Reviewed-by: Karl Heubaum <karl.heubaum@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>