pc: fix qemu exiting with error when -m X < 128 with old machine types

If machine doesn't support memory hotplug then starting QEMU
with initial memory less than default will make QEMU exit with
following error message:

$QEMU -m 16  -M isapc
qemu-system-i386: "-memory 'slots|maxmem'" is not supported by: isapc

Set maxram_size to initial memory value before parsing
'maxmem' option allows to keep maxmem in sync with initial
memory size if no maxmem option was specified.
Signed-off-by: NIgor Mammedov <imammedo@redhat.com>
CC: Bruce Rogers <brogers@suse.com>
Reviewed-By: NBruce Rogers <brogers@suse.com>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

vl.c

@@ -3315,6 +3315,7 @@ int main(int argc, char **argv, char **envp)
                     error_report("ram size too large");
                     exit(EXIT_FAILURE);
                 }
+                maxram_size = ram_size;
 
                 maxmem_str = qemu_opt_get(opts, "maxmem");
                 slots_str = qemu_opt_get(opts, "slots");