detect-virt: dmi: look for KVM

Some guests (ARM, AArch64, x86-RHEL) have 'KVM' in the product name.
Look for that first in order to more precisely report "kvm" when
detecting a QEMU/KVM guest. Without this patch we report "qemu",
even if KVM acceleration is in use on ARM/AArch64 guests.

I've only tested a backported version of this and the previous
patch on an AArch64 guest (which worked). Of course it would be
nice to get regression testing on all guest types that depend on
dmi done.