restore-test: add rootfs override alongside bind-mount overrides

PVE refuses a restore carrying mountpoint params unless rootfs is also set ("mount
points configured, but 'rootfs' not set"). Size the rootfs override from the source
rootfs (rootfsSizeGB/sizeToGB, round up). Validated manually on the host: restore of
bind-mounted 9201 with --rootfs + --mp0/--mp9 overrides boots + tears down cleanly.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/reconcile/restoretest_mounts_test.go

@@ -46,3 +46,19 @@ func TestBindMountOverrides(t *testing.T) {
 		t.Error("expected nil when there are no bind mounts")
 	}
 }
+
+func TestRootfsSizeGB(t *testing.T) {
+	cases := map[string]int{
+		"local-lvm:vm-9201-disk-0,size=8G": 8,
+		"local-lvm:base,size=512M":         1, // rounds up
+		"local-lvm:base,size=1T":           1024,
+		"local-lvm:base,size=0":            0,
+		"local-lvm:base":                   0, // no size field
+		"local-lvm:base,size=8":            8, // unit-less → GB
+	}
+	for spec, want := range cases {
+		if got := rootfsSizeGB(spec); got != want {
+			t.Errorf("rootfsSizeGB(%q) = %d, want %d", spec, got, want)
+		}
+	}
+}