Eliminate all uses of virBufferAdd with string literals.

Eliminate all uses of virBufferAdd with string literals.

* Makefile.maint (sc_prohibit_virBufferAdd_with_string_literal):
New rule.
* src/buf.h (virBufferAddLit): Define.
* src/conf.c (virConfSaveValue): Use virBufferAddLit, in place
of virBufferAdd everywhere possible.
(virConfSaveEntry): Likewise.
* src/qemu_conf.c (qemudGenerateXML, qemudGenerateNetworkXML): Likewise.
* src/qemu_driver.c (qemudGetFeatures, qemudGetCapabilities): Likewise.
* src/test.c (testDomainDumpXML, testNetworkDumpXML): Likewise.
* src/xen_internal.c (xenHypervisorMakeCapabilitiesXML): Likewise.
* src/xend_internal.c (xend_parse_sexp_desc_os): Likewise.
(xend_parse_sexp_desc, sexpr_to_xend_topology_xml): Likewise.
* src/xm_internal.c (xenXMDomainFormatXML, xenXMDomainPinVcpu): Likewise.
* src/xml.c (virSaveCpuSet, virParseXenCpuTopology): Likewise.
(virDomainParseXMLGraphicsDescImage): Likewise.
(virDomainParseXMLGraphicsDescVFB, virDomainParseXMLOSDescHVM): Likewise.
(virDomainParseXMLOSDescPV, virDomainParseXMLDiskDesc): Likewise.
(virDomainParseXMLIfDesc, virDomainParseXMLDesc): Likewise.