During qemuConnectGetAllDomainStats if qemuDomainGetStats causes
a failure, then when collecting more than one domain's worth of
statistics the loop in virDomainStatsRecordListFree would call
virDomainFree which would call virResetLastError effectively wiping
out the reason we failed leaving the caller with no idea why the
collection failed.

To fix this, let's Preserve the error and Restore it prior to return
so that a caller such as 'virsh domstats' doesn't get the generic
"error: An error occurred, but the cause is unknown".
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>