As qtest_availble() returns 1 only when CONFIG_POSIX is set, keep
setting AccelClass.available to keep current behavior (this is different
from what we did for KVM and Xen).

This also allows us to make qtest_init_accel() static.

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