char: convert qemu_chr_fe_write to qemu_chr_fe_write_all

The mux chardev was not checking the return value of any
qemu_chr_fe_write() call so would silently loose data
on EAGAIN.

Similarly the qemu_chr_fe_printf method would not check
errors and was not in a position to retry even if it
could check.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1473170165-540-5-git-send-email-berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
