KVM: Use return value for error print

Commit 94ccff13 introduced a more verbose failure message and retry
operations on KVM VM creation. However, it ended up using a variable
for its failure message that hasn't been initialized yet.

Fix it to use the value it meant to set.

Cc: qemu-stable@nongnu.org
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
