assembler/qemu: we want the boot fs type for grub2

The filesystem module that grub2 needs to have in the core image
is the filesystem containing the grub modules, specifically the
"normal.mod", as well as the grub configuration. In the standard
case, which is also what osbuild uses, this is /boot/grub2; thus
we actually do want the filesystem containing that directory and
its type not the root filesystem.

assemblers/org.osbuild.qemu

@@ -248,6 +248,14 @@ class PartitionTable:
                 return p
         return None
 
+    def partition_containing_boot(self) -> Partition:
+        """Return the partition containing /boot"""
+        for p in self.partitions_with_filesystems():
+            if p.mountpoint == "/boot":
+                return p
+        # fallback to the root partition
+        return self.partition_containing_root()
+
     def write_to(self, target, sync=True):
         """Write the partition table to disk"""
         # generate the command for sfdisk to create the table
@@ -341,19 +349,21 @@ def install_grub2(image: str, pt: PartitionTable):
     # read the partition and its filesystem containing said modules
     # and the grub configuration [NB: efi systems work differently]
 
+    # find the partition containing /boot/grub2
+    boot_part = pt.partition_containing_boot()
+
     # modules: access the disk and read the partition table
     modules = ["biosdisk", "part_msdos"]
 
-    # modules: grubs needs to access the filesystems
-    root_part = pt.partition_containing_root()
-    root_fs_type = root_part.fs_type or "unknown"
+    # modules: grubs needs to access the filesystems of /boot/grub2
+    fs_type = boot_part.fs_type or "unknown"
 
-    if root_fs_type == "ext4":
+    if fs_type == "ext4":
         modules += ["ext2"]
-    elif root_fs_type == "xfs":
+    elif fs_type == "xfs":
         modules += ["xfs"]
     else:
-        raise ValueError(f"unknown root filesystem type: '{root_fs_type}'")
+        raise ValueError(f"unknown boot filesystem type: '{fs_type}'")
 
     # now created the core image
     subprocess.run(["grub2-mkimage",