blockjob: introduce block_job_do_yield

Hide the clearing of job->busy in a single function, and set it
in block_job_enter.  This lets block_job_do_yield verify that
qemu_coroutine_enter is not used while job->busy = false.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Tested-By: Jeff Cody <jcody@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
