qemu: add "-boot strict" to commandline whenever possible

qemu: add "-boot strict" to commandline whenever possible

This resolves:

  https://bugzilla.redhat.com/show_bug.cgi?id=888635

(which was already closed as CANTFIX because the qemu "-boot strict"
commandline option wasn't available at the time).

Problem: you couldn't have a domain that used PXE to boot, but also
had an un-bootable disk device *even if that disk wasn't listed in the
boot order*, because if PXE timed out (e.g. due to the bridge
forwarding delay), the BIOS would move on to the next target, which
would be the unbootable disk device (again - even though it wasn't
given a boot order), and get stuck at a "BOOT DISK FAILURE, PRESS ANY
KEY" message until a user intervened.

The solution available since sometime around QEMU 1.5, is to add
"-boot strict=on" to *every* qemu command. When this is done, if any
devices have a boot order specified, then QEMU will *only* attempt to
boot from those devices that have an explicit boot order, ignoring the
rest.