swtpm: Abstract NVRAM interface for pluggable state store

swtpm: Abstract NVRAM interface for pluggable state store

Related GH issue: https://github.com/stefanberger/swtpm/issues/461

This patch adds an abstraction layer to NVRAM state store implementation
for pluggable storage backends. No functional changes are intended in
this change. The default state store backend ("dir backend") keeps
current behavior.

To make swtpm ready for pluggable store, this patch moves file related
operations to the seperate file (swtpm_nvstore_dir.c) and defined the
interface for plugins (nvram_backend_ops in swtpm_nvstore.h). The
interface can be used by each plugin which will be added later.

With the interface, each plugin can access its "backend_uri" which
points to the location of the backend storage, for example S3 bucket
URL or iSCSI URL, and decide how it stores TPM state data.

Signed-off-by: Eiichi Tsukata <eiichi.tsukata@nutanix.com>