virDomainChrGetDomainPtrs() required 4 levels of pointers (taking
a parameter that will be used as an output variable to return the
address of another variable that contains an array of pointers).
This is rather complex to reason about, especially when outside
of the domain_conf file, no other caller should be modifying
the resulting array of pointers directly.  Changing the public
signature gives something is easier to reason with, and actually
make const-correct; which is important as it was the only function
that was blocking virDomainDeviceDefCopy from treating its source
as const.

* src/conf/domain_conf.h (virDomainChrGetDomainPtrs): Use simpler
types, and make const-correct for external users.
* src/conf/domain_conf.c (virDomainChrGetDomainPtrs): Split...
(virDomainChrGetDomainPtrsInternal): ...into an internal version
that lets us modify terms, vs. external form that is read-only.
(virDomainDeviceDefPostParseInternal, virDomainChrFind)
(virDomainChrInsert): Adjust callers.
* src/qemu/qemu_command.c (qemuGetNextChrDevIndex): Adjust caller.
(qemuDomainDeviceAliasIndex): Make const-correct.
Signed-off-by: NEric Blake <eblake@redhat.com>