work around dynamic auto-ro for live migration
work around dynamic auto-ro for live migration

QEMU 4.0 introduced dynamic auto-read-only for file-backed drives. Since
then, live migration will break, when the qemu process does not run as
root (aka security_model=user|pool):
==> /var/log/ganeti/kvm/test.vm.log <==
kvm: Could not open '/var/run/ganeti/instance-disks/test.vm:0':
     Permission denied

Disable auto-read-only if supported by the -blockdev help message. This
allows migrations back and forth between qemu versions that support
auto-read-only and those that do not. See also commit
6d6adbb7187b4130c74e877e114222f039d35e11 which solves a similar problem
for hotpluged disks.

Signed-off-by: Sascha Lucas <159eafbe094024c3a34197db2452d0e30bd5531d@web.de>

ganeti/ganeti,ganeti/ganeti,ganeti/ganeti