Enhance Bootloader with optional 'set prefix='

In distros like Almalinux 10, GRUB modules are installed in
/usr/lib/grub in the rootfs. For GRUB to find them, a proper prefix has to
be set, which we can identify using its UUID.

This patch adds a new optional attribute to `Bootloader` object for
this, which is a `dict` with two keys:

- `uuid`: the UUID of the rootfs
- `path`: the directory in the rootfs where modules will be searched for

This is meant to be usable by host-installer in relevant contexts.

Signed-off-by: Yann Dirson <yann.dirson@vates.tech>

Author: ydirson

xcp/bootloader.py

@@ -86,7 +86,7 @@ class Bootloader(object):
     # pylint: disable=too-many-positional-arguments
     def __init__(self, src_fmt, src_file, menu = None, menu_order = None,
                  default = None, timeout = None, serial = None,
-                 location = None, env_block = None):
+                 location = None, env_block = None, prefix = None):
 
         if menu is None:
             menu = {}
@@ -104,6 +104,7 @@ def __init__(self, src_fmt, src_file, menu = None, menu_order = None,
         self.serial = serial
         self.location = location and location or 'mbr'
         self.env_block = env_block
+        self.prefix = prefix
 
     def append(self, label, entry):
         self.menu[label] = entry
@@ -297,6 +298,9 @@ def writeGrub2(self, dst_file = None):
         else:
             fh = open_textfile(cast(str, dst_file), "w")
 
+        if self.prefix:
+            print(f"search --no-floppy --fs-uuid --set=rootdev {self.prefix['uuid']}", file=fh)
+            print(f"set prefix=($rootdev){self.prefix['path']}", file=fh)
         if self.serial:
             print("serial --unit=%s --speed=%s" % (self.serial['port'],
                                                    self.serial['baud']), file=fh)