increase max supported NVMe request size (#6783)

The current default value for the NVMe MDTS parameter exposed in QEMU
emulated NMVe devices is 7 (max 512KiB requests). However there seems to
be an internal Windows Server 2025 issue that possibly triggers when
splitting bigger requests into smaller on in the NVMe Windows driver.

Increase the exposed MDTS value on the emulated QEMU NVMe device to 9
(max 2MiB request size), as that seems to drop the reproduction rate of
the issue.

Discussion is ongoing with Microsoft to get the issue identified and
possibly sorted on their end. For the time being apply this mitigation
in qemu-wrapper as a workaround.

Author: lindig

ocaml/xenopsd/xc/device.ml

@@ -2774,7 +2774,7 @@ module Backend = struct
         ]
 
       (* 4 and 5 are NICs, and we can only have two, 6 is platform *)
-      let extra_args = ["-device"; "nvme,serial=nvme0,id=nvme0,addr=7"]
+      let extra_args = ["-device"; "nvme,serial=nvme0,mdts=9,id=nvme0,addr=7"]
     end
 
     module XenPV = struct let addr ~xs:_ ~domid:_ _ ~nics:_ = 6 end