tests: Make the test_virtio_pmem test use a temporary file

Rather than using a raw OS disk image. This will be useful when the test
is extended to doing I/O on the image.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

Cargo.toml

@@ -33,6 +33,7 @@ dirs = "2.0.2"
 credibility = "0.1.3"
 tempdir= "0.3.7"
 lazy_static= "1.4.0"
+tempfile = "3.1.0"
 
 [features]
 default = ["acpi", "pci", "cmos"]