x86emul: conditionally clear BNDn for branches

x86emul: conditionally clear BNDn for branches

Considering that we surface MPX to HVM guests, instructions we emulate
should also correctly deal with MPX state. While for now BND*
instructions don't get emulated, the effect of branches (which we do
emulate) without BND prefix should be taken care of.

No need to alter XABORT behavior: While not mentioned in the SDM so
far, this restores BNDn as they were at the XBEGIN, and since we make
XBEGIN abort right away, XABORT in the emulator is only a no-op.

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