Add macOS disk resize support

[balcsida]

What the resize feature does is that when a guest VM starts the next time, before the guest OS boots, VirtualBuddy inspects the guest disk image by temporarily attaching it on the host with hdiutil and reading the partition layout. If it finds an APFS-on-GPT layout, it treats it as a macOS guest disk and checks for locked APFS volumes / FileVault first:

• If the disk is encrypted, the resize is blocked. I couldn't find a reliable way to resize FileVault-protected APFS offline, even after trying the recovery key in multiple formats. I may revisit this later.
• If the disk is not encrypted, the disk image is expanded to the requested size. Then diskutil expands the GPT layout and the APFS container to match. If APFS does not pick up the new ceiling on the first pass, which sometimes happens, the resizer applies a small shrink-and-grow nudge to force APFS to recompute the available space.