screenshot: don't unlink bogus file

screenshot: don't unlink bogus file

The previous qemu patch could end up calling unlink(tmp) before
tmp was the name of a valid file (unlinking a fileXXXXXX template
instead), or calling unlink(tmp) twice on success (once here,
and once at the end of the stream).  Meanwhile, vbox also suffered
from the same leaked tmp file bug.

* src/qemu/qemu_driver.c (qemuDomainScreenshot): Don't unlink on
success, or on invalid name.
* src/vbox/vbox_tmpl.c (vboxDomainScreenshot): Don't leak temp file.