This is the desired behavior for HMP, but it is a better choice for QMP as well.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

This failed on the new assertion of qemu_set_fd_handler2:

qemu-system-x86_64: /home/pbonzini/work/upstream/qemu/iohandler.c:60: qemu_set_fd_handler2: Assertion `fd >= 0' failed.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Adding an NBD server inside QEMU is trivial, since all the logic is
in nbd.c and can be shared easily between qemu-nbd and QEMU itself.
The main difference is that qemu-nbd serves a single unnamed export,
while QEMU serves named exports.
Acked-by: NLuiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>