qemuFirmwareFillDomain: Use FW descriptors to lookup template when resetting NVRAM

qemuFirmwareFillDomain: Use FW descriptors to lookup template when resetting NVRAM

If VIR_QEMU_PROCESS_START_RESET_NVRAM flag is passed when
starting a domain, then user requested to overwrite the domain
specific NVRAM with the one from template. But it is very likely
that the path to the template is not stored in the domain
definition, which in turn makes the copy function
(qemuPrepareNVRAM()) fail.

The solution is simple - when preparing domain, specifically when
deciding whether the path to the template should be autofilled,
ignore any existing NVRAM file.

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