main-loop: check return value before using pointer

pointer 'qemu_aio_context' should be checked first before it is used.
qemu_bh_new() will use it.

Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com>
Message-Id: <1467799740-26079-2-git-send-email-caoj.fnst@cn.fujitsu.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
