kola/testiso: set the VM's ram to value of '--qemu-memory' when provided

Currently `testiso` only uses 4Gb and ignores `--qemu-memory`, which leads
to a failure when testing new live-images with `/root.erofs`:
```
[    2.133469] dracut-cmdline[536]: /usr/sbin/initqueue: line 65: echo: write error: No space left on device
```

With this PR it's now possible to override default settings:
```
$ cosa kola testiso pxe* --pxe-append-rootfs --qemu-memory 8192
Running test: pxe-offline-install.bios
PASS: pxe-offline-install.bios (1m27.545s)
...
```

mantle/cmd/kola/testiso.go

@@ -27,6 +27,7 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"strconv"
 	"strings"
 	"time"
 
@@ -391,6 +392,14 @@ func newBaseQemuBuilder(outdir string) (*platform.QemuBuilder, error) {
 		builder.ConsoleFile = filepath.Join(outdir, "console.txt")
 	}
 
+	if kola.QEMUOptions.Memory != "" {
+		parsedMem, err := strconv.ParseInt(kola.QEMUOptions.Memory, 10, 32)
+		if err != nil {
+			return nil, err
+		}
+		builder.MemoryMiB = int(parsedMem)
+	}
+
 	return builder, nil
 }