kvm tools: Modify term_putc to write more than one char

It is a performance enhancement. When running in a simulator, each
system call to write a character takes a lot of time.  Batching them
up decreases the overhead (in the root kernel) of each virtio console
write.

Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: Andreas Herrmann <andreas.herrmann@caviumnetworks.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>