test: replace deprecated "fedora-13" machine with "pc-0.13"

test: replace deprecated "fedora-13" machine with "pc-0.13"

One of the xml tests in the test suite was created using a
now-deprecated qemu machine type ("fedora-13", which was only ever
valid for Fedora builds of qemu). Although strictly speaking it's not
necessary to replace it with an actual supported qemu machine type
(since the xml in question is never actually sent to qemu), this patch
changes it to the actually-supported "pc-0.13" just for general
tidiness. (Also, on some Fedora builds which contain a special patch
to rid the world of "fedora-13", having it mentioned in the test suite
will cause make check to fail.)