build: hoist qemu dependence on yajl to configure

build: hoist qemu dependence on yajl to configure

Commit 6e769eba made it a runtime error if libvirt was compiled
without yajl support but targets a new enough qemu.  But enough
users are hitting this on self-compiled libvirt that it is worth
erroring out at compilation time, rather than an obscure failure
when trying to use the built executable.

* configure.ac: If qemu is requested and -version works, require
yajl when qemu version is new enough.
* src/qemu/qemu_capabilities.c (qemuCapsComputeCmdFlags): Add
comment.