Here a very short howto
virsh shutdown kvm11111
qemu-img resize hd.qcow2 +10G
fdisk /dev/vda
Command (m for help): p
Command (m for help): d
Command (m for help): n
Do you want to remove the signature? [Y]es/[N]o: N
Command (m for help): w
boot a rescue image then run
e2fsck -f /dev/vda1
resize2fs /dev/vda1
Have fun!