src/launch: improve the addition of the no-hpet option

Since HPET is specific to x86, we can safely add it its option only on
x86 and x86_64 when creating the libvirt XML (no more hitting the
launching failures due to that on other architectures).

Regarding the direct qemu launch, since qemu 1.1 (which is our current
minimum) "-ho-hpet" appears in the help only where actually supported,
so we could just checking for it and adding it only if present. This
should fix the architecture issues on this backend as well.