qemu: Introduce qemuDomainSecretHostdevPrepare and Destroy

qemu: Introduce qemuDomainSecretHostdevPrepare and Destroy

Similar to the qemuDomainSecretDiskPrepare, generate the secret
for the Hostdev's prior to call qemuProcessLaunch which calls
qemuBuildCommandLine. Additionally, since the secret is not longer
added as part of building the command, the hotplug code will need
to make the call to add the secret in the hostdevPriv.

Since this then is the last requirement to pass a virConnectPtr
to qemuBuildCommandLine, we now can remove that as part of these
changes. That removal has cascading effects through various callers.

Signed-off-by: John Ferlan <jferlan@redhat.com>