qemu: Get/return compressedpath program

qemu: Get/return compressedpath program

Based upon a patch from Chen Hanxiao <chenhanxiao@gmail.com>, rather than
need to call virFindFileInPath twice, let's just save the path and pass it
along with the compressed type. (NB: the second call would be in virExec as
called from virCommandRunAsync which is called from qemuMigrationToFile
using the argument 'compressor' which up to this point would be the string
from the cfg file that isn't the fully qualified path).

Since we now have the path, we can remove qemuCompressProgramName which
would return NULL or the string representation of the compress type.