detect error when kernel and initrd images exceed ram size

I was unable to figure out why my VM wasn't booting when I added a "-initrd" image.  I would launch qemu and get no output, and no error message, it would just spin.

Turns out my initrd image was around 270 MB but I wasn't giving an explicit ram size to qemu.  I was told the default memory size was around 120 MB so this was definitely a problem.  I think that the qemu "pseudo-bootloader" should detect when the kernel image and initrd image sizes exceed the size of ram and print a nice error to the user, something like:

Error: the total size of the given boot images (342M) exceeds the size allocated for memory (120M)