monitor: accept input on resume

monitor: accept input on resume

A chardev may stop trying to write if the associated can_read()
callback returned 0. This happens when the monitor is suspended.
The frontend is supposed to call qemu_chr_fe_accept_input() when it is
ready to accept data again.

An issue was observed with a spice port: pending commands may be
delayed, as the chardev is not flushed. Most chardev don't use the
accept_input() callback, and instead check regularly if they can
write. The ones that do use it are braille, mux, msmouse,
spice (abstract), spicevmc, spiceport, wctablet.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180817173752.19136-1-marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>