config: Detect if VM memory smaller than image

Add a heuristic to ensure the amount of memory allocated to the
hypervisor is bigger than the size of the image.

This catches simple configuration issues where `default_memory=` is set
to a smaller value than the size of either the `image=` or `initrd=`
files.

If the configured image type is `initrd`, fail but only warn in the
logs for `image` as although it seems a highly unlikely scenario, it is
permitted.

Update tests to ensure that created resources have `>0` bytes.

Fixes #636.

Signed-off-by: James O. D. Hunt <james.o.hunt@intel.com>

cli/utils.go

@@ -13,6 +13,7 @@ import (
 	"os/exec"
 	"path/filepath"
 	"strings"
+	"syscall"
 )
 
 const (
@@ -230,3 +231,15 @@ func writeFile(filePath string, data string, fileMode os.FileMode) error {
 func isEmptyString(b []byte) bool {
 	return len(bytes.Trim(b, "\n")) == 0
 }
+
+// fileSize returns the number of bytes in the specified file
+func fileSize(file string) (int64, error) {
+	st := syscall.Stat_t{}
+
+	err := syscall.Stat(file, &st)
+	if err != nil {
+		return 0, err
+	}
+
+	return st.Size, nil
+}