KVM: x86: Use LVT_TIMER instead of an open coded literal

Use LVT_TIMER instead of the literal '0' to clean up the apic_lvt_mask
lookup when emulating handling writes to APIC_LVTT.

No functional change intended.

Signed-off-by: Liam Ni <zhiguangni01@gmail.com>
[sean: manually regenerate patch (whitespace damaged), massage changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>