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: Igor Mammedov <imammedo@redhat.com>
CC: Bruce Rogers <brogers@suse.com>
Reviewed-By: Bruce Rogers <brogers@suse.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>