vmspawn: fix QEMU target names for ppc64/mips64el/mipsel hosts

vmspawn: fix QEMU target names for ppc64/mips64el/mipsel hosts

architecture_to_qemu_table mapped four 64-bit/LE architectures to wrong
qemu-system-<target> names: MIPS64_LE and MIPS_LE both to "mips" (the BE
MIPS32 target), and PPC64/PPC64_LE to "ppc" (32-bit PowerPC). The string
feeds both find_qemu_binary() ("qemu-system-" + name) and the
firmware.json architecture match, so on those hosts vmspawn looked up a
wrong or nonexistent binary and never matched a firmware descriptor. Map
them to the real targets: mips64el, mipsel, and ppc64 (qemu-system-ppc64
runs LE pSeries guests; there is no qemu-system-ppc64le).

Co-developed-by: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Paul Meyer <katexochen0@gmail.com>