vl: split various early command line options to a separate function

Various options affect the global state of QEMU including the rest of
qemu_init, and they need to be called very early.  Group them together
in a function that is called at the beginning.

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