Update config-4.6 and config-4.5 to point to stable branches; fix config-master

Update config-4.6 and config-4.5 to point to stable branches; fix config-master

Point xen, qemu, and qemu-trad to stable-4.5 and -4.6 branches.

And point the default libvirt to point to the libvirt 1.3.3
maintenance branch, rather than xen-tested-master.

Also update OVMF revision for 4.6 to a version that builds with modern
gccs.

Point config-master libvirt to libvirt's "master" rather than xenbits'
xen-tested-master, to avoid having to switch to xenbits repo.

Finally, fix config-master by pointing ovmf and seabios to the correct
branch (master, not xen-tested-master).

Singed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
---
v2:
 - Use libvirt-master rather than xen-tested-master for config-master
 - Fix seabios and ovmf revisions for config-master

CC: Stefano Stabellini <sstabellini@kernel.org>