Adds <smartcard mode='passthrough' type='spicevmc'/>, which uses the
new <channel name='smartcard'/> of <graphics type='spice'>.

* docs/schemas/domain.rng: Support new XML.
* docs/formatdomain.html.in: Document it.
* src/conf/domain_conf.h (virDomainGraphicsSpiceChannelName): New
enum value.
(virDomainChrSpicevmcName): New enum.
(virDomainChrSourceDef): Distinguish spicevmc types.
* src/conf/domain_conf.c (virDomainGraphicsSpiceChannelName): Add
smartcard.
(virDomainSmartcardDefParseXML): Parse it.
(virDomainChrDefParseXML, virDomainSmartcardDefParseXML): Set
spicevmc name.
(virDomainChrSpicevmc): New enum conversion functions.
* src/libvirt_private.syms: Export new functions.
* src/qemu/qemu_command.c (qemuBuildChrChardevStr): Conditionalize
name.
* tests/qemuxml2argvtest.c (domain): New test.
* tests/qemuxml2argvdata/qemuxml2argv-smartcard-passthrough-spicevmc.args:
New file.
* tests/qemuxml2argvdata/qemuxml2argv-smartcard-passthrough-spicevmc.xml:
Likewise.