KVM: x86: Consolidate PIT legacy test

Move the test for KVM_PIT_FLAGS_HPET_LEGACY into create_pit_timer
instead of replicating it on the caller site.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>