enhance importer to use qemu-img pseudo streaming to save /tmp step

[jeffvance]

For qcow2 files, rather than copying to /tmp/rand-name and then running qcow2-img to convert it to raw, we can use qcow2-img to copy the file and convert it, saving the intermediate step and saving disk/cpu on the host. Of course .tar.gz, .xz conversion needs to still be done first so this may only help for the simple case of importing .qcow2 --> raw.

Some timings from a recent email:

$ time wget http://local.server/Fedora-AtomicHost-28-20180625.1.x86_64.qcow2 
...
Length: 638043136 (608M) [application/octet-stream]Saving to: ‘Fedora-AtomicHost-28-20180625.1.x86_64.qcow2’

Fedora-AtomicHost-28-20180625.1.x86_64.qcow2    100%[=====================================================================================================>] 608.49M   107MB/s    in 5.6s    

2018-07-09 21:38:39 (108 MB/s) - ‘Fedora-AtomicHost-28-20180625.1.x86_64.qcow2’ saved [638043136/638043136]


real	0m5.941s
user	0m0.183s
sys	0m1.185s

$ time qemu-img convert -p -f qcow2 -O raw http://local.server/Fedora-AtomicHost-28-20180625.1.x86_64.qcow2 converted.raw
    (100.00/100%)

real	0m14.217s
user	0m5.235s
sys	0m2.343s

$ time qemu-img convert -p -f qcow2 -O raw Fedora-AtomicHost-28-20180625.1.x86_64.qcow2 converted.raw
    (100.00/100%)

real	0m11.909s
user	0m4.728s
sys	0m1.595s

So converting on the fly is even little faster then downloading to temporary file and converting.