Qemu has two operating mode named full system emulation and user mode emulation. If you want to simulate whole system not just the cpu (like a PC) you need to use full system emulation mode.
Newer distributions have separate binaries for these two different operation modes. For example, if you just want user mode cpu emulation for X86-64 architecture, you need to use qemu-x86_64 binary and if you need whole X86-64 bit system emulation (like your PC), qemu-system-x86_64 binary must be used in commands below.
You have an iso image and you want to boot from it without restarting the system, simply use qemu virtual machine as below (
-m 512 says qemu will be use 512 Mb of RAM from system):
qemu-system-x86_64 -boot d -cdrom image.iso -m 512
It is also possible to use your regular cdrom device too. If the device is
/dev/cdrom you can boot a cd in the device like that:
qemu-system-x86_64 -boot d -cdrom /dev/cdrom -m 512
Above examples not use any harddisk, so it is suitable for live cd image case. If you want to install a distribution to a harddisk image file, you need to create harddisk image file first:
qemu-img create mydisk.img 10G
In this example we're created an image of 10 GB. Now we can use this file as harddisk in our qemu boot:
qemu-system-x86_64 -boot d -cdrom image.iso -m 512 -hda mydisk.img