quorum: Add quorum_open() and quorum_close().

Example of command line:
-drive if=virtio,file.driver=quorum,\
file.children.0.file.filename=1.raw,\
file.children.0.node-name=1.raw,\
file.children.0.driver=raw,\
file.children.1.file.filename=2.raw,\
file.children.1.node-name=2.raw,\
file.children.1.driver=raw,\
file.children.2.file.filename=3.raw,\
file.children.2.node-name=3.raw,\
file.children.2.driver=raw,\
file.vote_threshold=2

file.blkverify=on with file.vote_threshold=2 and two files can be passed to
emulated blkverify.

Signed-off-by: Benoit Canet <benoit@irqsave.net>
quorum: Add unit test.

Signed-off-by: Benoit Canet <benoit@irqsave.net>