QEMU warns when msize is <= 8192 due to degraded performance.
This change bumps our default msize to 128 KiB, matching the
Linux Kernel v5.15 and newer default. Linux supports even larger values,
but 128 KiB is a sensible default.
We also add a new 'msize' mount option to allow users to override
this value, and we validate it against our maximum supported MTU
(currently fixed by the UMA zone size).