KVM: VMX: Reset DR6 only when KVM_DEBUGREG_WONT_EXIT

The commit efdab992 ("KVM: x86: fix escape of guest dr6 to the host")
fixed a bug by resetting DR6 unconditionally when the vcpu being scheduled out.

But writing to debug registers is slow, and it can be visible in perf results
sometimes, even if neither the host nor the guest activate breakpoints.

Since KVM_DEBUGREG_WONT_EXIT on Intel processors is the only case
where DR6 gets the guest value, and it never happens at all on SVM,
the register can be cleared in vmx.c right after reading it.

Reported-by: Lai Jiangshan <laijs@linux.alibaba.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>