qemu: Use qemuGetCompressionProgram for error paths

qemu: Use qemuGetCompressionProgram for error paths

Let's do some more code reuse - there are 3 other callers that care to
check/get the compress program. Each of those though cares whether the
requested cfg image is valid and exists. So, add a parameter to handle
those cases.

NB: We won't need to initialize the returned value in the case where
the cfg image doesn't exist since the called program will handle that.