qsp: track BQL callers explicitly

qsp: track BQL callers explicitly

The BQL is acquired via qemu_mutex_lock_iothread(), which makes
the profiler assign the associated wait time (i.e. most of
BQL wait time) entirely to that function. This loses the original
call site information, which does not help diagnose BQL contention.
Fix it by tracking the callers explicitly.

Signed-off-by: Emilio G. Cota <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>