Optimize I/O settings on lizard

Originally created by @intrigeri on #11817 (Redmine)

• I/O scheduler = noop in KVM guests
• I/O scheduler on the host: “The cfq scheduler is not recommended for hardware that does not suffer a large seek penalty, such as […] solid state disks.” ⇒ elevator=deadline for SSDs, or “5.3.5.1. Tuning cfq for fast storage” https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html-single/Performance_Tuning_Guide/
• libvirt/QEMU I/O mode:
  • It seems that we want <driver name='qemu' type='raw' io='native'/> according to https://www.ibm.com/support/knowledgecenter/en/linuxonibm/liaat/liaatbpkvmasynchio.htm and http://specs.openstack.org/openstack/nova-specs/specs/mitaka/implemented/libvirt-aio-mode.html.
  • https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html-single/Virtualization_Tuning_and_Optimization_Guide/index.html#sect-Virtualization_Tuning_Optimization_Guide-BlockIO-IO_mode

Related issues

• Blocks tails/sysadmin#11179