qemu-sockets: improve error reporting in unix_listen_opts

Coverity complains about not checking the returned value of mkstemp.  While
at it, also improve error checking for snprintf, and refine error messages
in general.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
