qemu: Change return value of SaveImageGetCompressionProgram

qemu: Change return value of SaveImageGetCompressionProgram

qemuSaveImageGetCompressionProgram is a bit overloaded. Along with
getting a compression program, it checks the validity of the image
format and returns the integer representation of the format. Change
the function to only handle retrieving the specified compression
program, returning success or failure. Checking the validity of
the image format can be left to the calling functions.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>