qemu: validate: Fix logic for validating presence of the HPET timer

qemu: validate: Fix logic for validating presence of the HPET timer

Commit 24cc9cda826 switched over to use -machine hpet, but one of the
steps it did was to clear the QEMU_CAPS_NO_HPET capability.

The validation check still uses the old capability though which means
that for configs which would explicitly enable HPET we'd report an error.

Since HPET is an x86(_64) platform specific device, convert the
validation check to an architecture check as all supported qemu versions
actually support it.

Modify a test case to request HPET to catch posible future problems.

Fixes: 24cc9cda826
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>