KVM: x86: Rename kvm_set_msi_irq() => kvm_msi_to_lapic_irq()

KVM: x86: Rename kvm_set_msi_irq() => kvm_msi_to_lapic_irq()

Rename kvm_set_msi_irq() to kvm_msi_to_lapic_irq() to better capture what
it actually does, e.g. it's _really_ easy to conflate kvm_set_msi_irq()
with kvm_set_msi().

Opportunistically delete the public declaration and export, as they are
no longer used/needed.

No functional change intended.

Link: https://lore.kernel.org/r/20250611224604.313496-64-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>