Include termios.h as POSIX fallback when not glibc, bsd or solaris.
POSIX says that termios.h should define struct termios and TCAFLUSH.
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/termios.h.html

This fixes the following compile errors with musl libc:

util/qemu-openpty.c: In function 'qemu_openpty_raw':
util/qemu-openpty.c:112:20: error: storage size of 'tty' isn't known
     struct termios tty;
                    ^
...
util/qemu-openpty.c:128:24: error: 'TCSAFLUSH' undeclared (first use in this function)
     tcsetattr(*aslave, TCSAFLUSH, &tty);
                        ^
Signed-off-by: NNatanael Copa <ncopa@alpinelinux.org>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>