profiling in user mode.

This change should allow reasonable and accurate user-mode profiling.
An MCP can look at its vcore stats and see at what time (in ns)
it was resumed. It can also see see total ns spent in that process.
Finally, it can compute the current ns spent in the process
by converting the tsc to ns and subtracting the resume value.

Only a proposal, let me know if there's a better way.

Signed-off-by: Ronald G. Minnich <rminnich@google.com>
Profiling: counters for the vcore.

Set a timer (resume) to the ns when a vcore resumes user mode.
This is almost certainly being done at the wrong place.

When a vcore exits user mode, update the total ns timer for that
vcore.

These two counters should let a user figure out profiling info.

Signed-off-by: Ronald G. Minnich <rminnich@google.com>