Check for invalid initrd file

When qemu is invoked with an invalid initrd file, it crashes. Following
patch prints a error message and exits if an invalid initrd is
specified. Includes changes suggested by JV.

Signed-off-by: M. Mohan Kumar <mohan@in.ibm.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
