KVM: x86: fix root cause for missed hardware breakpoints

Commit 172b2386 ("KVM: x86: fix missed hardware breakpoints",
2016-02-10) worked around a case where the debug registers are not loaded
correctly on preemption and on the first entry to KVM_RUN.

However, Xiao Guangrong pointed out that the root cause must be that
KVM_DEBUGREG_BP_ENABLED is not being set correctly.  This can indeed
happen due to the lazy debug exit mechanism, which does not call
kvm_update_dr7.  Fix it by replacing the existing loop (more or less
equivalent to kvm_update_dr0123) with calls to all the kvm_update_dr*
functions.

Cc: stable@vger.kernel.org   # 4.1+
Fixes: 172b2386
Reviewed-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>