lxd/instance/drivers/driver/qemu: Adds trans=virtio to 9p mounts
lxd/instance/drivers/driver/qemu: Adds trans=virtio to 9p mounts

Otherwise CentOS 7 VM guests can't mount 9p shares.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

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