only set LIBGUESTFS_HV for ppc64le on el7

[dustymabe]

It appears the newer P9 hardware that we are utilizing that is running a Fedora stack don't need this and it's causing other errors like:

libguestfs: error: could not create appliance through libvirt.

Try running qemu directly without libvirt using this environment variable:
export LIBGUESTFS_BACKEND=direct

Original error from libvirt: internal error: process exited while connecting to monitor: 2022-08-10T19:06:20.395168Z qemu-system-ppc64: The -accel and "-machine accel=" options are incompat
ible [code=1 int1=-1]

This could partially be because when you execute the qemu process through LIBGUESTFS_HV that the env variable LIBGUESTFS_BACKEND doesn't get passed through to it? I'm not sure but let's hack around this for now while we still have el7 clusters.