KVM: x86: Drop kvm_x86_ops.set_dr6() in favor of a new KVM_RUN flag

KVM: x86: Drop kvm_x86_ops.set_dr6() in favor of a new KVM_RUN flag

Instruct vendor code to load the guest's DR6 into hardware via a new
KVM_RUN flag, and remove kvm_x86_ops.set_dr6(), whose sole purpose was to
load vcpu->arch.dr6 into hardware when DR6 can be read/written directly
by the guest.

Note, TDX already WARNs on any run_flag being set, i.e. will yell if KVM
thinks DR6 needs to be reloaded.  TDX vCPUs force KVM_DEBUGREG_AUTO_SWITCH
and never clear the flag, i.e. should never observe KVM_RUN_LOAD_GUEST_DR6.

Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20250610232010.162191-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>