Do not add HPET timer config to non x86 targets

HPET is a harware timer for x86 arch. Also, qemu option '-no-hpet'
is not supported on non x86 targets. This change does not add hpet
in the guest's timer config when the arch is not x86_64 or i686.

Closes-Bug: #1290362

Change-Id: I1b81d1c5ed4f900ed87d1ecbbb97d4fdb22405fa
Merge "Do not add HPET timer config to non x86 targets"