snapshot: minor cleanups from reviewing indentation

snapshot: minor cleanups from reviewing indentation

Break some long lines, and use more efficient functions when possible,
such as relying on virBufferEscapeString to skip output on a NULL arg.
Ensure that output does not embed newlines, since auto-indent won't
work in those situations.

* src/conf/domain_conf.c (virDomainTimerDefFormat): Break output lines.
(virDomainDefFormatInternal, virDomainDiskDefFormat)
(virDomainActualNetDefFormat, virDomainNetDefFormat)
(virDomainHostdevDefFormat): Minor cleanups.