pc: reduce duplication, fix PIIX descriptions

pc: reduce duplication, fix PIIX descriptions

We have a lot of code duplication between machine types,
this increases with each new machine type
and each new field.

This has already introduced a minor bug: description
for pc-1.3 says "Standard PC" while description for
pc-1.4 is "Standard PC (i440FX + PIIX, 1996)"
which makes you think 1.3 is somehow more standard,
or newer, while in fact it's a revision of the same PC.

This patch addresses this issue by using macros, along
the lines used by PC_COMPAT_X_X - only for
non-property options.

The approach can extend to non-PC machine types.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>