We need qemu-6.1 for ACPI PCI hotplug support for the q35 machine. At the
moment qemu will use SHPC hotplug under the PCIe to PCI bridge on q35.
SHPC is too slow to use for our purposes (it requires a 5s delay).
Update the qemu version to v6.1.0. This leaves the experimental version
*older* than the normal version, but we'll fix that up later.
We also need to tweak the snapcraft.yaml, since the location for configs
has changed in the new qemu version.
fixes#1691
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>