Do not initialize Sound for the build VMs

QEMU by default initializes sound access, which is unnecessary
(we do not want to have sound devices, ever) and also hurts
performance on armv7l - the probing and spilling error messages
takes a bit of startup time.
Various KVM related fixes for armv7l

We do not want to hardcode the memsize but instead use
the host configuration. Also prefer the guest initrd
over the host initrd, as they might differ (and do)
Merge pull request #51 from dirkmueller/bug/armv7_fixes

Bug/armv7 fixes