qemu: Re-add driver unlock to qemuDomainSendKey

Should have been done in commit 56fd513 already, but was missed
due to oversight: qemuDomainSendKey didn't release the driver lock
in its cleanup section. This fixes an issue introduced by commit
8c5d2ba.

Signed-off-by: Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com>

src/qemu/qemu_driver.c

@@ -2439,6 +2439,7 @@ endjob:
 cleanup:
     if (vm)
         virObjectUnlock(vm);
+    qemuDriverUnlock(driver);
     return ret;
 }