maint: use lighter-weight function for straight appends

maint: use lighter-weight function for straight appends

It costs quite a few processor cycles to go through printf parsing
just to determine that we only meant to append.

* src/xen/xend_internal.c (xend_op_ext): Consolidate multiple
printfs into one.
* src/qemu/qemu_command.c (qemuBuildWatchdogDevStr)
(qemuBuildUSBInputDevStr, qemuBuildSoundDevStr)
(qemuBuildSoundCodecStr, qemuBuildVideoDevStr): Likewise.
(qemuBuildCpuArgStr, qemuBuildCommandLine): Prefer virBufferAdd
over virBufferVsprintf for trivial appends.
* src/phyp/phyp_driver.c (phypExec, phypUUIDTable_Push)
(phypUUIDTable_Pull): Likewise.
* src/conf/nwfilter_conf.c (macProtocolIDFormatter)
(arpOpcodeFormatter, formatIPProtocolID, printStringItems)
(virNWFilterPrintStateMatchFlags, virNWIPAddressFormat)
(virNWFilterDefFormat): Likewise.
* src/security/virt-aa-helper.c (main): Likewise.
* src/util/sexpr.c (sexpr2string): Likewise.
* src/xenxs/xen_sxpr.c (xenFormatSxprChr): Likewise.
* src/xenxs/xen_xm.c (xenFormatXMDisk): Likewise.