pass info about hpets to seabios.]

Currently HPET ACPI table is created regardless of whether qemu actually
created hpet device. This may confuse some guests that don't check that
hpet is functional before using it. Solve this by passing info about
hpets in qemu to seabios via fw config interface. Additional benefit is
that seabios no longer uses hard coded hpet configuration. Proposed
interface supports up to 8 hpets. This is the number defined by hpet
spec.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
