libxc: clearer migration v2 debug message

libxc: clearer migration v2 debug message

Previous the message was like:

SAVE:
xc: detail: 32 bits, 3 levels
xc: detail: max_pfn 0xfffff, p2m_frames 1024
xc: detail: max_mfn 0x130000

RESTORE:
xc: detail: max_mfn 0x130000
xc: detail: 32 bits, 3 levels
xc: detail: Expanded p2m from 0 to 0xfffff

It's not immediately clear that the last line in restore messages was
referring to max_pfn. Change the debug message a bit to make that
clearer.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>