Reload the LDT selector after an AMD-v #VMEXIT.

cpu_switch() always reloads the LDT, so this can only affect the
hypervisor process itself. Fix this by explicitly reloading the host
LDT selector after each #VMEXIT. The stock bhyve process on FreeBSD
never uses a custom LDT, so this change is cosmetic.

Reviewed by: kib
Tested by: Mike Tancsa <mike@sentex.net>
Approved by: re (gjb)
MFC after: 2 weeks