i386: fix '-cpu ?' output for host cpu type

Since commit d6dcc558, '-cpu ?' shows the description of the
X86_CPU_TYPE_NAME("max") for the host CPU model:

Enables all features supported by the accelerator in the current host

instead of the expected:

KVM processor with all supported host features

or

HVF processor with all supported host features

This is caused by the early use of kvm_enabled() and hvf_enabled() in
a class_init function. Since the accelerator isn't configured yet, both
helpers return false unconditionally.

A QEMU binary will only be compiled with one of these accelerators, not
both. The appropriate description can thus be decided at build time.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <153055056654.212317.4697363278304826913.stgit@bahia.lan>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>