target-i386: Allow physical address bits to be set

Currently QEMU sets the x86 number of physical address bits to the
magic number 40.  This is only correct on some small AMD systems;
Intel systems tend to have 36, 39, 46 bits, and large AMD systems
tend to have 48.

Having the value different from your actual hardware is detectable
by the guest and in principal can cause problems;
The current limit of 40 stops TB VMs being created by those lucky
enough to have that much.

This patch lets you set the physical bits by a cpu property but
defaults to the same 40bits which matches TCGs setup.

I've removed the ancient warning about the 42 bit limit in exec.c;
I can't find that limit in there and no one else seems to know where
it is.

We use a magic value of 0 as the property default so that we can
later distinguish between the default and a user set value.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
