podman machine: image override files need user-write access

Issue Description

While doing some testing today, I learned that at least for qemu machines, qemu-img requires user write access or we get an error. This is probably true for all providers but we need to check that.

❯ podman machine init --image builds/latest/x86_64/rhcos-10.1.20251017-0-qemu.x86_64.qcow2 test
Extracting compressed file: test-amd64.qcow2: done  
qemu-img: Could not open '/home/baude/.local/share/containers/podman/machine/qemu/test-amd64.qcow2': Could not open '/home/baude/.local/share/containers/podman/machine/qemu/test-amd64.qcow2': Permission denied
Error: resizing image: "exit status 1"

and

❯ ls -l builds/latest/x86_64/rhcos-10.1.20251017-0-qemu.x86_64.qcow2
-r--r--r--. 1 baude baude 1950744576 Nov 20 15:21 builds/latest/x86_64/rhcos-10.1.20251017-0-qemu.x86_64.qcow2

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podman info output

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Podman in a container

No

Privileged Or Rootless

None

Upstream Latest Release

Yes

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