"info mtree -f" prints the wrong accelerator name if used with for example
"-machine accel=kvm:tcg".  The right thing to do is to fetch the name
from the AccelClass, which will also work nicely once
current_machine->accel stops existing.

Tested-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>