qtest: pre-buffer hex nibs

Instead of converting each byte one-at-a-time and then sending each byte
over the wire, use sprintf() to pre-compute all of the hex nibs into a
single buffer, then send the entire buffer all at once.

This gives a moderate speed boost to memread() and memwrite() functions.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 1431021095-7558-2-git-send-email-jsnow@redhat.com