qemu-char: retry g_poll on EINTR

This is a case where pty_chr_update_read_handler_locked's lack
of error checking can produce incorrect values.  We are not using
SIGUSR1 anymore, so this is quite theoretical, but easy to fix.

Reported-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
