xapi_xenops: Split update_vm internals into a separate function

No functional change, this just removes several indentation levels from the
500+ lines of the function, making it easier to refactor in the future.

This also clears up the logic of the function, now that two arms of if-else and
try-with clauses are not 500+ lines apart, and avoids splitting some
expressions and strings over several lines given that they reach the line
character limit more often inside of several levels of indentation.

Signed-off-by: Andrii Sultanov <[email protected]>

Author: last-genius