qemu: Don't ignore dump image format errors

qemu: Don't ignore dump image format errors

Long ago, without justification, commit 48cb9f0542 changed
qemuGetCompressionProgram (since renamed to
qemuSaveImageGetCompressionProgram) to ignore configuration errors
for dump operations. Like the other save-related operations, user
provided configuration should be verified and an error reported if
it cannot be honored.

Remove the special handling of configuration errors in
qemuSaveImageGetCompressionProgram and change the dump logic to
fail when dump image format cannot be supported.

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