qtest: kill QEMU process on g_assert() failure

The QEMU process stays running if the test case fails.  This patch fixes
the leak by installing a SIGABRT signal handler which invokes
qtest_end().

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
