G'day, I don't know how much you know about networking and QEMU/KVM, or whether you're using user mode networking or another mode, but: if you don't specify what sort of networking, you get user mode. User mode allows you to connect to external TCP/UDP resources, but no other protocols (e.g. you can't ping), and acts like a firewall.
There's a reasonably simple summary of networking in QEMU on Wikibooks
, with more information on the #qemu Wiki
. The Wikibooks page tells you how to set up incoming ports with user mode networking, and also how to set up a TAP interface for full networking capabilities if you require that.