dnsmasq: Fix errno handling and don't unlink non-existing files

dnsmasq: Fix errno handling and don't unlink non-existing files

addnhostsSave and hostsfileSave expect < 0 return value on error from
addnhostsWrite and hostsfileWrite but then pass err instead of -err
to virReportSystemError that expects an errno value.

Also addnhostsWrite returns -ENOMEM and errno, change this to -errno.

addnhostsWrite and hostsfileWrite tried to unlink the tempfile after
renaming it, making both fail on the final step. Remove the unnecessary
unlink calls.