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>
