typedparams.go: Propagate error when virTypedParamsGet* helpers fail

So far, when unpacking typed parameters only successful case is considered. Neither of virTypedParamsGet* wrappers are checked for error and even when they set one, this is just thrown away. This also leads to a memory leak because the err variable is never freed.

Signed-off-by: Michal Privoznik mprivozn@redhat.com