pc: fix regression for 64 bit PCI memory

commit 398489018183d613306ab022653552247d93919f
    pc: limit 64 bit hole to 2G by default
introduced a way for management to control
the window allocated to the 64 bit PCI hole.

This is useful, but existing management tools do not know how to set
this property.  As a result, e.g. specifying a large ivshmem device with
size > 4G is broken by default.  For example this configuration no
longer works:

-device ivshmem,size=4294967296,chardev=cfoo
-chardev socket,path=/tmp/sock,id=cfoo,server,nowait

Fix this by detecting that hole size was not specified
and defaulting to the backwards-compatible value of 1 << 62.

Cc: qemu-stable@nongnu.org
Cc: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
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.

Note: this conflicts (trivially) with patch
    [PATCH v3 6/6] hw: Clean up bogus default boot order
by Markus. If people like this approach, I can rebase
on top of that patch and apply both.

The approach can extend to non-PC machine types.

Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>