virsh list [--all] lists out running domains (add '--all' to list domains which are not running)
virsh reboot [DOMAIN] issues reboot command to domain
qemu-img resize image.qcow2 +1G resize disk image by 1 gigabyte. You will also need to resize the partitions in the virtual disk in order to use the new space.
This can also be used to shrink a qcow image.
Ensure that the partitions on the disk are shrunk using in OS options, gparted for example
qemu-img resize image.qcow2 -1G This will shrink the disk image by 1 gigabyte.