tpm: Move qemuTPMEmulatorInit to virTPMEmulatorInit in virtpm.c

tpm: Move qemuTPMEmulatorInit to virTPMEmulatorInit in virtpm.c

Move qemuTPMEmulatorInit to virTPMEmulatorInit in virtpm.c and introduce
a few functions to query the executables needed for virCommands.

Add locking to protect the tool paths and return a copy of the tool paths
to callers wanting to access them so that we can run the initialization
function multiples time later on and detect when the executable gets updated.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>