[snapshots] Snapshot and update QEMU mount args

[ricab]

Keep reference to mount args in snapshot and update when applying. I am still not sure if this is necessary, since the QemuMountHandler updates the mount args upon construction and destruction.

[codecov]

Codecov Report

Merging #3151 (ab0a8a9) into fix-qemu-restore (efa7342) will decrease coverage by 0.04%.
The diff coverage is 21.73%.

@@                 Coverage Diff                  @@
##           fix-qemu-restore    #3151      +/-   ##
====================================================
- Coverage             84.89%   84.86%   -0.04%     
====================================================
  Files                   246      246              
  Lines                 13001    13006       +5     
====================================================
  Hits                  11037    11037              
- Misses                 1964     1969       +5     

Impacted Files	Coverage Δ
src/platform/backends/qemu/qemu_snapshot.cpp	2.32% <0.00%> (-0.31%)	⬇️
src/platform/backends/qemu/qemu_virtual_machine.h	100.00% <ø> (ø)
...rc/platform/backends/qemu/qemu_virtual_machine.cpp	66.88% <35.71%> (ø)

[ricab]

I couldn't find a use case that needed this after all. If we restored snapshots on suspended instances, we would need this because. But we don't, so closing until we find some other reason.