target-i386: clear bsp bit when designating bsp

Since the BSP bit is writable on real hardware, during reset all the CPUs which
were not chosen to be the BSP should have their BSP bit cleared. This fix is
required for KVM to work correctly when it changes the BSP bit.

An additional fix is required for QEMU tcg to allow software to change the BSP
bit.

Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Message-Id: <1427932716-11800-1-git-send-email-namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>