Currently all qemu driver tests are statically linked to qemu driver
library, which makes it impossible to mock any API from the library.
This patch creates a shared qemu driver library which can be used
instead of the static one.

NB we can't use libvirt_driver_qemu.so directly since it is linked with
-module and it is supposed to be dlopened.
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>