KVM: VMX: Don't send UNBLOCK when starting device assignment without APICv

KVM: VMX: Don't send UNBLOCK when starting device assignment without APICv

When starting device assignment, i.e. potential IRQ bypass, don't blast
KVM_REQ_UNBLOCK if APICv is disabled/unsupported.  There is no need to
wake vCPUs if they can never use VT-d posted IRQs (sending UNBLOCK guards
against races being vCPUs blocking and devices starting IRQ bypass).

Opportunistically use kvm_arch_has_irq_bypass() for all relevant checks in
the VMX Posted Interrupt code so that all checks in KVM x86 incorporate
the same information (once AMD/AVIC is given similar treatment).

Cc: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://lore.kernel.org/r/20250401161804.842968-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>