Let's fix this before we bake in a painful API.  Since we know
that we have exactly one non-negative fd on success, we might
as well return the fd directly instead of forcing the user to
pass in a pointer.  Furthermore, I found some memory and fd
leaks while reviewing the code - the idea is that on success,
libvirtd will have handed two fds in two different directions:
one to qemu, and one to the RPC client.

* include/libvirt/libvirt.h.in (virDomainOpenGraphicsFD): Drop
unneeded parameter.
* src/driver.h (virDrvDomainOpenGraphicsFD): Likewise.
* src/libvirt.c (virDomainOpenGraphicsFD): Adjust interface to
return fd directly.
* daemon/remote.c (remoteDispatchDomainOpenGraphicsFd): Adjust
semantics.
* src/qemu/qemu_driver.c (qemuDomainOpenGraphicsFD): Likewise,
and plug fd leak.
* src/remote/remote_driver.c (remoteDomainOpenGraphicsFD):
Likewise, and plug memory and fd leak.
Signed-off-by: NEric Blake <eblake@redhat.com>