accel: Move KVM accel registration to kvm-all.c

accel: Move KVM accel registration to kvm-all.c

Note that this has an user-visible side-effect: instead of reporting
"KVM is not supported for this target", QEMU binaries not supporting KVM
will report "kvm accelerator does not exist".

As kvm_availble() always return 1 when CONFIG_KVM is enabled, we don't
need to set AccelClass.available anymore. kvm_enabled() is not being
completely removed yet only because qmp_query_kvm() still uses it.

This also allows us to make kvm_init() static.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>