vl.c: fix regression when reading memory size from config file

vl.c: fix regression when reading memory size from config file

This is happening because an actual logic is performed on the memory
arguments inside the main's switch, disregarding the config file content.

Solved by extracting the logic on a separate function and calling it
after the switch.

Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>