hw/pci-host/raven: Fix crash when introspecting raven-pcihost from the CLI

QEMU currently crashes when introspecting raven-pcihost from the command
line interface:

 $ ./qemu-system-ppc -device raven-pcihost,help
 Segmentation fault (core dumped)

This happens because the raven_pcihost_initfn instance init function
calls get_system_memory(), but that is not available here yet.

There does not seem to be a compelling reason for initializing the
memory regions from the instance init function, so let's simply move
the code into the realize() function instead to fix this issue.

Tested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20260317074049.436460-1-thuth@redhat.com>