build: fix race with creating qapi-generated

Since qapi-generated/ is a global QEMU include path, we need to make
sure it is created before anything is compiled, so do this in the
configure phase rather than via the Makefile.

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
