ivshmem: Fix 64 bit memory bar configuration

Device ivshmem property use64=0 is designed to make the device
expose a 32 bit shared memory BAR instead of 64 bit one.  The
default is a 64 bit BAR, except pc-1.2 and older retain a 32 bit
BAR.  A 32 bit BAR can support only up to 1 GiB of shared memory.

This worked as designed until commit 5400c02 accidentally flipped
its sense: since then, we misinterpret use64=0 as use64=1 and vice
versa.  Worse, the default got flipped as well.  Devices
ivshmem-plain and ivshmem-doorbell are not affected.

Fix by restoring the test of IVShmemState member not_legacy_32bit
that got messed up in commit 5400c02.  Also update its
initialization for devices ivhsmem-plain and ivshmem-doorbell.
Without that, they'd regress to 32 bit BARs.

Cc: qemu-stable@nongnu.org
Signed-off-by: Zhuang Yanying <ann.zhuangyanying@huawei.com>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
