x86emul: work around SandyBridge errata

x86emul: work around SandyBridge errata

There are a number of exception condition related errata on SandyBridge
CPUs, some of which are unexpected #UD (others, of no interest here, are
lack of mandated exceptions, or exceptions of unexpected type). Annotate
the one workaround we already have, and add two more.

Due to the exception recovery we have in place for stub invocations
these aren't security issues.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>