virt-install: create disk image without an explicit path

For most of its life, virt-install has required specifying an explicit disk path when creating storage, like:

virt-install --disk /path/to/my/new/disk.img,size=10 ...

However there's a shortcut since version 1.0.0, just specify the size:

virt-install --disk size=10 ...

virt-install will create a disk image in the default pool, and name it based on the VM name and disk image format, typically $vmname.qcow2.