vl: remove separate preconfig main_loop

Move post-preconfig initialization to the x-exit-preconfig.  If preconfig
is not requested, just exit preconfig mode immediately with the QMP
command.

As a result, the preconfig loop will run with accel_setup_post
and os_setup_post restrictions (xen_restrict, chroot, etc.)
already done.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>