vl: move prelaunch part of qemu_init to new functions

The final part of qemu_init, starting with the completion of
board init, is already relatively clean.  Split it out of
qemu_init so that qemu_init keeps only the messy parts.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>