curl: convert readv to coroutines

This is pretty simple.  The bottom half goes away because, unlike
bdrv_aio_readv, coroutine-based read can return immediately without
yielding.  However, for simplicity I kept the former bottom half
handler in a separate function.

Reviewed-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20170515100059.15795-7-pbonzini@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>