monitor: simplify functions for getting a dup'd fdset entry

Currently code has to call monitor_fdset_get_fd, then dup
the return fd, and then add the duplicate FD back into the
fdset. This dance is overly verbose for the caller and
introduces extra failure modes which can be avoided by
folding all the logic into monitor_fdset_dup_fd_add and
removing monitor_fdset_get_fd entirely.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

include/qemu/osdep.h

@@ -501,6 +501,7 @@ int qemu_open(const char *name, int flags, ...);
 int qemu_close(int fd);
 int qemu_unlink(const char *name);
 #ifndef _WIN32
+int qemu_dup_flags(int fd, int flags);
 int qemu_dup(int fd);
 #endif
 int qemu_lock_fd(int fd, int64_t start, int64_t len, bool exclusive);