target-i386: Set physical address bits based on host

Add the host-phys-bits boolean property, if true, take phys-bits
from the hosts physical bits value, overriding either the default
or the user specified value.

We can also use the value we read from the host to check the users
explicitly set value and warn them if it doesn't match.

Note:
   a) We only read the hosts value in KVM mode (because on non-x86
      we get an abort if we try)
   b) We don't warn about trying to use host-phys-bits in TCG mode,
      we just fall back to the TCG default.  This allows the machine
      type to set the host-phys-bits flag if it wants and then to
      work in both TCG and KVM.

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>
