vm_tools: sommelier: Split out functions from real_main

Sommelier's main() function is really large (595 lines). This CL splits
out functionality for opening a Wayland socket, running a parent
instance, and spawning XWayland into separate functions. This makes the
main() function a bit easier to follow, and allows re-using the code for
opening a Wayland socket (which will be needed for cros_im work).

Logic has not been changed but some variable declarations have been
moved closer to their first usage.

BUG=b:232048569
TEST=manual: build as per crostini_developer_guide.md, briefly use VS
Code and gedit

Change-Id: Ib529de196820bec8a098ecd341bfc4dce92cb4a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform2/+/3877833
Tested-by: Timothy Loh <[email protected]>
Reviewed-by: Nic Hollingum <[email protected]>
Commit-Queue: Timothy Loh <[email protected]>