memory hotplug: rework max memory handling, make phys-bits dependent

memory hotplug: rework max memory handling, make phys-bits dependent

QEMU 7.1 introduced some actual checks for the max memory value in
1caab5cf86bd ("i386/pc: bounds check phys-bits against max used GPA")
and while correct it breaks our by-luck working hard coded max mem of
4 TB for cases with smaller phys bit address sizes, like some older
CPUs or most CPU types have per default if not 'host' or 'max'.

QEMU uses 40 bits per default if the CPU isn't set to host or
phys-bits is not set explicitly.

For 40 bit it seems that depending on machine type one has a max
possible mem of: i440 -> 752, q35 -> 722 GiB, but instead of reducing
it to 704 GiB (512+1128+64) in a hard coded way we acutally check for
the bit size that will probably be used and use that to determine the
max memory size useable.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>