qemu: Make qemuMigrationSrcCancel optionally synchronous

qemu: Make qemuMigrationSrcCancel optionally synchronous

We have always considered "migrate_cancel" QMP command to return after
successfully cancelling the migration. But this is no longer true (to be
honest I'm not sure it ever was) as it just changes the migration state
to "cancelling". In most cases the migration is canceled pretty quickly
and we don't really notice anything, but sometimes it takes so long we
even get to clearing migration capabilities before the migration is
actually canceled, which fails as capabilities can only be changed when
no migration is running. So to avoid this issue, we can wait for the
migration to be really canceled after sending migrate_cancel. The only
place where we don't need synchronous behavior is when we're cancelling
migration on user's request while it is actively watched by another
thread.

https://bugzilla.redhat.com/show_bug.cgi?id=2114866

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>