KVM: vmx: add support for emulating UMIP

UMIP can be emulated almost perfectly on Intel processor by enabling
descriptor-table exits.  SMSW does not cause a vmexit and hence it
cannot be changed into a #GP fault, but all in all it's the most
"innocuous" of the unprivileged instructions that UMIP blocks.

In fact, Linux is _also_ emulating SMSW instructions on behalf of the
program that executes them, because some 16-bit programs expect to use
SMSW to detect vm86 mode, so this is an even smaller issue.

Reviewed-by: Wanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>