User emulator builds do not have error_report() so it should not be used
by simpletrace.c. In fact, error reporting inside simpletrace.c is
inappropriate and should be done by the caller instead.
This patch moves st_init() error reporting out to its caller,
vl.c:main().
Reported-by: Blue Swirl <blauwirbel@gmail.com> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com> Signed-off-by: Blue Swirl <blauwirbel@gmail.com>