[macOS] Only half the number of CPUs is available for instances in the GUI

[ricab]

Describe the bug

The maximum number of CPUs that the user can select for an instance in the GUI is limited to 4, even though I have 8 hardware threads available:

(Same when trying to configure existing instances.)

And yet:

$ nproc
8
$ sysctl -n hw.ncpu
8

Additional info

• OS: macOS 15.1
• multipass version: 1.15.0-rc4
• multipass get local.driver: both VirtualBox an QEMU

[Sploder12]

This is also the case for Windows

[georgeliao]

Indeed, and this seem be fine on the linux.

[ricab]

Fixed by #3867