Reenable write support for VMDK etc. in tools

Kevin Wolf requested to merge kmwolf/centos-qemu-kvm:bz1989841 into c9s

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1989841 Brew: https://brewweb.engineering.redhat.com/brew/taskinfo?taskID=38976017

Turns out that Red Hat actually needs support for VMDK and VHD as target formats for 'qemu-img convert' for Image Builder. Reenable the block drivers on the read-only whitelist for unrestricted use in tools to unblock these users.

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