vl.c: move -m parsing after memory backends has been processed

It will be possible for main RAM to come from memory-backend
and we should check that size specified in -m matches the size
of the backend and [MachineState::]ram_size also matches
backend's size.

However -m parsing (set_memory_options()) happens before backends
are intialized (object_create_delayed()) which complicates it.
Consolidate set_memory_options() and assigning parsed results to
current_machine after backends are initialized, so it would be
possible access the initialized backend instance to compare
sizes.

This patch only consolidates scattered places touching ram_size
within vl.c. And follow up patch will integrate backend handling
to set_memory_options().

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20200219160953.13771-7-imammedo@redhat.com>