start: allow discarding managed save

There have been several instances of people having problems with
a broken managed save file, and not aware that they could use
'virsh managedsave-remove dom' to fix things.  Making it possible
to do this as part of starting a domain makes the same functionality
easier to find, and one less API call.

* include/libvirt/libvirt.h.in (VIR_DOMAIN_START_FORCE_BOOT): New
flag.
* src/libvirt.c (virDomainCreateWithFlags): Document it.
* src/qemu/qemu_driver.c (qemuDomainObjStart): Alter signature.
(qemuAutostartDomain, qemuDomainStartWithFlags): Update callers.
* tools/virsh.c (cmdStart): Expose it in virsh.
* tools/virsh.pod (start): Document it.

include/libvirt/libvirt.h.in

@@ -236,6 +236,7 @@ typedef enum {
     VIR_DOMAIN_START_PAUSED       = 1 << 0, /* Launch guest in paused state */
     VIR_DOMAIN_START_AUTODESTROY  = 1 << 1, /* Automatically kill guest when virConnectPtr is closed */
     VIR_DOMAIN_START_BYPASS_CACHE = 1 << 2, /* Avoid file system cache pollution */
+    VIR_DOMAIN_START_FORCE_BOOT   = 1 << 3, /* Boot, discarding any managed save */
 } virDomainCreateFlags;