save: support bypass-cache flag in qemu.conf

save: support bypass-cache flag in qemu.conf

When auto-dumping a domain on crash events, or autostarting a domain
with managed save state, let the user configure whether to imply
the bypass cache flag.

* src/qemu/qemu.conf (auto_dump_bypass_cache, auto_start_bypass_cache):
Document new variables.
* src/qemu/libvirtd_qemu.aug (vnc_entry): Let augeas parse them.
* src/qemu/qemu_conf.h (qemud_driver): Store new preferences.
* src/qemu/qemu_conf.c (qemudLoadDriverConfig): Parse them.
* src/qemu/qemu_driver.c (processWatchdogEvent, qemuAutostartDomain):
Honor them.