maint: simplify lots of libxml2 clients

maint: simplify lots of libxml2 clients

Repetitive patterns should be factored.  The sign of a good
factorization is a change that kills 5x more lines than it adds :)

* src/conf/domain_conf.c (virDomainDeviceDefParse)
(virDomainSnapshotDefParseString): Use new convenience macros.
* src/conf/storage_conf.c (virStoragePoolDefParseSourceString):
Likewise.
* src/cpu/cpu.c (cpuCompareXML, cpuBaselineXML): Likewise.
* src/esx/esx_vi.c (esxVI_Context_Execute): Likewise.
* src/qemu/qemu_migration.c (qemuMigrationCookieXMLParseStr):
Likewise.
* src/security/virt-aa-helper.c (caps_mockup): Likewise.
* src/test/test_driver.c (testOpenFromFile): Likewise.
* tests/cputest.c (cpuTestLoadXML, cpuTestLoadMultiXML):
Likewise.
* tools/virsh.c (cmdFreecell, makeCloneXML, cmdVNCDisplay)
(cmdTTYConsole, cmdDetachInterface, cmdDetachDisk)
(cmdSnapshotCreate, cmdSnapshotCreateAs, cmdSnapshotCurrent)
(cmdSnapshotList, cmdSnapshotParent): Likewise.