qemu.qemu_img:Fix convert.to_qed and to_raw test cases SKIP

Currently, qemu.convert.to_qed and qemu.convert.to_raw will
be skip, and the log is:

17:37:31 INFO | SKIP qcow2.virtio_blk.smp2.virtio_net.
Fedora.18.x86_64.qemu_img.convert.to_qed -> ParamNotFound:
Mandatory parameter 'compressed' is missing.
Check your cfg files for typos/mistakes

This is because in convert test, it will use params["compressed"]
and params["encrypted"], but for to_qed and to_raw, the two params
does not exist. In old version virttest, it will be OK, as it use
params.get("compressed"), and this will return 'None', the test can
run safely.

But in the new version, it will just SKIP due to no param 'compressed'.

This patch fixes problem with these two cases.

Signed-off-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>