vmm(4): Reset host LDTR on exit for SVM

vmm(4): Reset host LDTR on exit for SVM

For SVM machines, the LDT content remains set to that of the guest VM on
exit (as compared to Intel/VMX which resets the LDTR to 0). This fix
ensures the LDT is reset to 0 on SVM exits.

Leaving the LDT set to the guest's choice could allow a malicious process
to escalate its privileges with the help of a malicious VM that they
also are able to run on the machine.

This was reported by Maxime Villard; thanks!