target/i386: fix interrupt CPL error when using ist in x86-64

In do_interrupt64(), when interrupt stack table(ist) is enabled
and the the target code segment is conforming(e2 & DESC_C_MASK), the
old implementation always set new CPL to 0, and SS.RPL to 0.

This is incorrect for when CPL3 code access a CPL0 conforming code
segment, the CPL should remain unchanged. Otherwise higher privileged
code can be compromised.

The patch fix this for always set dpl = cpl when the target code segment
is conforming, and modify the last parameter `flags`, which contains
correct new CPL, in cpu_x86_load_seg_cache().

Signed-off-by: Wu Xiang <willx8@gmail.com>
Message-Id: <20170621142152.GA18094@wxdeubuntu.ipads-lab.se.sjtu.edu.cn>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
