snapshot: split snapshot conf code into own file

snapshot: split snapshot conf code into own file

This has several benefits:
1. Future snapshot-related code has a definite place to go (and I
_will_ be adding some)
2. Snapshot errors now use the VIR_FROM_DOMAIN_SNAPSHOT error
classification, which has been underutilized (previously only in
libvirt.c)

* src/conf/domain_conf.h, domain_conf.c: Split...
* src/conf/snapshot_conf.h, snapshot_conf.c: ...into new files.
* src/Makefile.am (DOMAIN_CONF_SOURCES): Build new files.
* po/POTFILES.in: Mark new file for translation.
* src/vbox/vbox_tmpl.c: Update caller.
* src/esx/esx_driver.c: Likewise.
* src/qemu/qemu_command.c: Likewise.
* src/qemu/qemu_domain.h: Likewise.