If a package supports only one implementation then the ssl use flag should be used. See thread at: http://archives.gentoo.org/gentoo-dev/msg_0673a33fe75961e510872fd2c1044ced.xml
I've updated the use flag for qemu-kvm-9999. Will take care of releases at a later date.
I've now updated the use flag for qemu-kvm-0.12.5-r1 as well.
This has now been fixed in ebuilds starting with qemu-kvm-0.12.5-r1. It won't be fixed for the older ebuilds which will simply be removed sooner or later.