qemu_tpm: Make APIs work over a single virDomainTPMDef

qemu_tpm: Make APIs work over a single virDomainTPMDef

In qemu_extdevice.c lives code that handles helper daemons that
are required for some types of devices (e.g. virtiofsd,
vhost-user-gpu, swtpm, etc.). These devices have their own
handling code in separate files, with only a very basic functions
exposed (e.g. for starting/stopping helper process, placing it
into given CGroup, etc.). And these functions all work over a
single instance of device (virDomainVideoDef *, virDomainFSDef *,
etc.), except for TPM handling code which takes virDomainDef *
and iterates over it inside its module.

Remove this oddness and make qemuExtTPM*() functions look closer
to the rest of the code.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>