xen: xen creates it's own UEFI bios from OVMF, which pulls in repos during build phase.

[refutationalist]

This is a badness per Arch package guidelines. There are multiple options for a solution, in increasing desirability:

• pull in needed sources as we do with the stubdom system
• spin OVMF for xen into it's own package
• politely ask the edk2-ovmf packager to include the Xen OVMF build

We're currently pursuing the third option.