qemu: turn if into switch in qemuDomainValidateDevicePCISlotsQ35
qemu: turn if into switch in qemuDomainValidateDevicePCISlotsQ35

This will make it simpler to add checks for other types of
controllers.

This is a prerequisite for patches to resolve:

   https://bugzilla.redhat.com/show_bug.cgi?id=1003983

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