spapr: compute default value of "hpt-max-page-size" later

spapr: compute default value of "hpt-max-page-size" later

It is currently not possible to run a pseries-2.12 or older machine
with HV KVM. QEMU prints the following and exits right away.

qemu-system-ppc64: KVM doesn't support for base page shift 34

The "hpt-max-page-size" capability was recently added to spapr to hide
host configuration details from HPT mode guests. Its default value for
newer machine types is 64k.

For backwards compatibility, pseries-2.12 and older machine types need
a different value. This is handled as usual in a class init function.
The default value is 16G, ie, all page sizes supported by POWER7 and
newer CPUs, but HV KVM requires guest pages to be hpa contiguous as
well as gpa contiguous. The default value is the page size used to
back the guest RAM in this case.

Unfortunately kvmppc_hpt_needs_host_contiguous_pages()->kvm_enabled() is
called way before KVM init and returns false, even if the user requested
KVM. We thus end up selecting 16G, which isn't supported by HV KVM. The
default value must be set during machine init, because we can safely
assume that KVM is initialized at this point.

We fix this by moving the logic to default_caps_with_cpu(). Since the
user cannot pass cap-hpt-max-page-size=0, we set the default to 0 in
the pseries-2.12 class init function and use that as a flag to do the
real work.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>