This patch enables the "none" USB controller for qemu guests and adds
valdiation on hot-plugged devices if the guest has USB disabled.

This patch also adds a set of tests to check parsing of domain XMLs that
use the "none" controller and some forbidden situations concerning it.

<vcpu> is not an optional node. The value for its 'placement'
actually always defaults to 'static' in the underlying codes.
(Even no 'cpuset' and 'placement' is specified, the domain
process will be pinned to all the available pCPUs).

The test domain allows <memory>0</memory>, but the RNG was stating
that memory had to be at least 4096000 bytes.  Hypervisors should
enforce their own limits, rather than complicating the RNG.

Meanwhile, some copy and paste had introduced some fishy constructs
in various unit tests.

* docs/schemas/domaincommon.rng (memoryKB, memoryKBElement): Drop
limit that isn't enforced in code.
* src/conf/domain_conf.c (virDomainDefParseXML): Require current
<= maximum.
* tests/qemuxml2argvdata/*.xml: Fix offenders.

Make it obvious to 'dumpxml' readers what unit we are using,
since our default of KiB for memory (1024) differs from qemu's
default of MiB; and differs from our use of bytes for storage.

Tests were updated via:

$ find tests/*data tests/*out -name '*.xml' | \
  xargs sed -i 's/<\(memory\|currentMemory\|hard_limit\|soft_limit\|min_guarantee\|swap_hard_limit\)>/<\1 unit='"'KiB'>/"
$ find tests/*data tests/*out -name '*.xml' | \
  xargs sed -i 's/<\(capacity\|allocation\|available\)>/<\1 unit='"'bytes'>/"

followed by a few fixes for the stragglers.

Note that with this patch, the RNG for <memory> still forbids
validation of anything except unit='KiB', since the code silently
ignores the attribute; a later patch will expand <memory> to allow
scaled input in the code and update the RNG to match.

* docs/schemas/basictypes.rng (unit): Add 'bytes'.
(scaledInteger): New define.
* docs/schemas/storagevol.rng (sizing): Use it.
* docs/schemas/storagepool.rng (sizing): Likewise.
* docs/schemas/domaincommon.rng (memoryKBElement): New define; use
for memory elements.
* src/conf/storage_conf.c (virStoragePoolDefFormat)
(virStorageVolDefFormat): Likewise.
* src/conf/domain_conf.h (_virDomainDef): Document unit used
internally.
* src/conf/storage_conf.h (_virStoragePoolDef, _virStorageVolDef):
Likewise.
* tests/*data/*.xml: Update all tests.
* tests/*out/*.xml: Likewise.
* tests/define-dev-segfault: Likewise.
* tests/openvzutilstest.c (testReadNetworkConf): Likewise.
* tests/qemuargv2xmltest.c (blankProblemElements): Likewise.

domain parsing and serialization code, qemu driver backend and
a couple of test

add a few tests for the new constructs

Assuming a hypervisor that supports multiple smartcard devices in the
guest, this would be a valid XML description:

<devices>
  <smartcard mode='host'/>
  <smartcard mode='host-certificates'>
    <certificate>/path/to/cert1</certificate>
    <certificate>/path/to/cert2</certificate>
    <certificate>/path/to/cert3</certificate>
  </smartcard>
  <smartcard mode='passthrough' type='tcp'>
    <source mode='bind' host='127.0.0.1' service='2001'/>
    <protocol type='raw'/>
  </smartcard>
</devices>

(As of this commit, the qemu hypervisor will be the first
implementation, but it only supports one smartcard.)

* docs/formatdomain.html.in (Smartcard devices): New section.
* docs/schemas/domain.rng (smartcard): New define, used in
devices.
* tests/qemuxml2argvdata/qemuxml2argv-smartcard-host.xml: New file
to test schema.
* tests/qemuxml2argvdata/qemuxml2argv-smartcard-host-certificates.xml:
Likewise.
* tests/qemuxml2argvdata/qemuxml2argv-smartcard-passthrough-tcp.xml:
Likewise.
* tests/qemuxml2argvdata/qemuxml2argv-smartcard-controller.xml:
Likewise.