acpi: build_hpet: use acpi_table_begin()/acpi_table_end() instead of build_header()

it replaces error-prone pointer arithmetic for build_header() API,
with 2 calls to start and finish table creation,
which hides offsets magic from API user.

while at it convert build_hpet() to endian agnostic
build_append_FOO() API

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20210924122802.1455362-15-imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>