KVM --disksize option to grow target disk image

[luke-jr]

Trying to build Talos II firmware in gitian, but hitting disk space issues.

For now, this lets the yml file choose a larger disk size. Only supported in KVM.

[devrandom]

utACK

needs rebase

[luke-jr]

Rebased, but changed it from a YML option to a command-line gbuild option, since it is primarily dependent on user cache size.