util: Report error in virFileWrapperFdClose()

util: Report error in virFileWrapperFdClose()

libvirt_iohelper is used internally by the virFileWrapperFd APIs;
more specifically, in the QEMU driver we have the doCoreDump() and
qemuDomainSaveMemory() helper functions as users, and those in turn
end up being called by the implementation of several driver APIs.

By calling virReportError() if libvirt_iohelper has failed, we
overwrite whatever generic error message QEMU might have raised
with the more useful one generated by the helper program.

After this commit, the user will be able to see the error directly
instead of having to dig in the journal or libvirtd log.

https://bugzilla.redhat.com/show_bug.cgi?id=1578741

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>