refactor: de-generify async_io engine

[roypat]

These generic type parameters were leaking out from the io_uring module. Having a generic ring structure in the io_uring module: makes sense. Having a generic file engine: probably makes less sense (what are we ever gonna process in there except block io requests?). Just replace all instances of T in the virtio module with PendingRequest - that's what it all ended up getting monomorphized to anyway.

Changes

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Reason

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[codecov]

Codecov Report

Attention: Patch coverage is 93.84615% with 4 lines in your changes missing coverage. Please review.

Project coverage is 83.09%. Comparing base (5eaa6e0) to head (dbf0c8c).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...vmm/src/devices/virtio/block/virtio/io/async_io.rs	93.75%	2 Missing ⚠️
src/vmm/src/devices/virtio/block/virtio/io/mod.rs	93.54%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5151      +/-   ##
==========================================
+ Coverage   83.04%   83.09%   +0.04%     
==========================================
  Files         250      250              
  Lines       26900    26892       -8     
==========================================
+ Hits        22340    22346       +6     
+ Misses       4560     4546      -14     

Flag	Coverage Δ
5.10-c5n.metal	83.59% <93.84%> (-0.01%)	⬇️
5.10-m5n.metal	83.59% <93.84%> (+<0.01%)	⬆️
5.10-m6a.metal	82.81% <93.84%> (-0.01%)	⬇️
5.10-m6g.metal	79.38% <93.84%> (-0.01%)	⬇️
5.10-m6i.metal	83.58% <93.84%> (+<0.01%)	⬆️
5.10-m7a.metal-48xl	82.79% <93.84%> (?)
5.10-m7g.metal	79.38% <93.84%> (-0.01%)	⬇️
5.10-m7i.metal-24xl	83.54% <93.84%> (?)
5.10-m7i.metal-48xl	83.54% <93.84%> (?)
5.10-m8g.metal-24xl	79.37% <93.84%> (?)
5.10-m8g.metal-48xl	79.37% <93.84%> (?)
6.1-c5n.metal	83.63% <93.84%> (-0.01%)	⬇️
6.1-m5n.metal	83.63% <93.84%> (-0.01%)	⬇️
6.1-m6a.metal	82.85% <93.84%> (-0.01%)	⬇️
6.1-m6g.metal	79.38% <93.84%> (-0.01%)	⬇️
6.1-m6i.metal	83.62% <93.84%> (-0.01%)	⬇️
6.1-m7a.metal-48xl	82.84% <93.84%> (?)
6.1-m7g.metal	79.38% <93.84%> (-0.01%)	⬇️
6.1-m7i.metal-24xl	83.64% <93.84%> (?)
6.1-m7i.metal-48xl	83.64% <93.84%> (?)
6.1-m8g.metal-24xl	79.37% <93.84%> (?)
6.1-m8g.metal-48xl	79.37% <93.84%> (?)

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