KVM: x86: fix escape of guest dr6 to the host

syzkaller reported:

   WARNING: CPU: 0 PID: 12927 at arch/x86/kernel/traps.c:780 do_debug+0x222/0x250
   CPU: 0 PID: 12927 Comm: syz-executor Tainted: G           OE    4.15.0-rc2+ #16
   RIP: 0010:do_debug+0x222/0x250
   Call Trace:
    <#DB>
    debug+0x3e/0x70
   RIP: 0010:copy_user_enhanced_fast_string+0x10/0x20
    </#DB>
    _copy_from_user+0x5b/0x90
    SyS_timer_create+0x33/0x80
    entry_SYSCALL_64_fastpath+0x23/0x9a

The testcase sets a watchpoint (with perf_event_open) on a buffer that is
passed to timer_create() as the struct sigevent argument.  In timer_create(),
copy_from_user()'s rep movsb triggers the BP.  The testcase also sets
the debug registers for the guest.

However, KVM only restores host debug registers when the host has active
watchpoints, which triggers a race condition when running the testcase with
multiple threads.  The guest's DR6.BS bit can escape to the host before
another thread invokes timer_create(), and do_debug() complains.

The fix is to respect do_debug()'s dr6 invariant when leaving KVM.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>