Fix guestfs custom-image path

After upgrading the CI host and my personal box to Debian forky, I run
into several custom image path issues. The following patches solves
them allowing to boot Debian trixie images on forky. While at it, add an
option to configure the VM size for both base and custom image paths.

# Describe the purpose of this series. The information you put here
# will be used by the project maintainer to make a decision whether
# your patches should be reviewed, and in what priority order. Please be
# very detailed and link to any relevant discussions or sites that the
# maintainer can review to better understand your proposed changes. If you
# only have a single patch in your series, the contents of the cover
# letter will be appended to the "under-the-cut" portion of the patch.

# Lines starting with # will be removed from the cover letter. You can
# use them to add notes or reminders to yourself. If you want to use
# markdown headers in your cover letter, start the line with ">#".

# You can add trailers to the cover letter. Any email addresses found in
# these trailers will be added to the addresses specified/generated
# during the b4 send stage. You can also run "b4 prep --auto-to-cc" to
# auto-populate the To: and Cc: trailers based on the code being
# modified.

To: Luis Chamberlain <[email protected]>
To: Chuck Lever <[email protected]>
Cc: [email protected]
Signed-off-by: Daniel Gomez <[email protected]>

--- b4-submit-tracking ---
# This section is used internally by b4 prep for tracking purposes.
{
  "series": {
    "revision": 1,
    "change-id": "20250820-guestfs-custom-image-fixes-ec6a8d1aa36f",
    "prefixes": []
  }
}

Author: dkruces