[9front] vm disk performance

[hiro <23hiro@gmail.com>]

i realized that my 9front KVM cwfs install assumes 512byte sectors.
the physical disk actually uses 4k sectors.

the partition table also aligns things by sector size.
until nvram we are aligned by big enough multiples of 512 that they
happen to also be multiples of 4k.
but nvram is one sector big, causing the next important partition
(other) to be misaligned by those 512bytes.

these 2 things together might affect performance considerably,
especially on hdds.

personally i also use zvols from zfs between the VMs and my disk, they
also happen to have a blocksize, the rather low default should
generally be increased to 16k to align with cwfs. even though i doubt
anybody else here is using zvols :)

i remember also a long time ago i discovered (on shady forums, don't
remember) that kvm on linux by default forces all normal (async)
writes to the virtual disk for "legacy" operating systems to by
synchronous to protect us from ourselves.
personally i disabled sync writes on my zfs as a good enough remedy,
but i have *no clue* how much better it might work if kvm could be
convinced we are a microsoft/redhat certified non-legacy system.

since i managed to find already 4 very probable big bottlenecks for
common setups with 9front inside VMs, i'd like to share my preliminary
view on this matter.

tests outstanding.