iscsi: do not use aio_context_acquire/release

iscsi: do not use aio_context_acquire/release

Now that all bottom halves and callbacks take care of taking the
AioContext lock, we can migrate some users away from it and to a
specific QemuMutex or CoMutex.

Protect libiscsi calls with a QemuMutex.  Callbacks are invoked
using bottom halves, so we don't even have to drop it around
callback invocations.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20170222180725.28611-4-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>