This introduces a hash table for qemu driver, to store the shared
disk's info as (@major:minor, @ref_count). @ref_count is the number
of domains which shares the disk.

Since we only care about if the disk support unprivileged SG_IO
commands, and the SG_IO commands only make sense for block disk,
this patch only manages (add/remove hash entry) the shared disk for
block disk.

* src/qemu/qemu_conf.h: (Add member 'sharedDisks' of type
                         virHashTablePtr; Declare helpers
                         qemuGetSharedDiskKey, qemuAddSharedDisk
                         and qemuRemoveSharedDisk)
* src/qemu/qemu_conf.c (Implement the 3 helpers)
* src/qemu/qemu_process.c (Update 'sharedDisks' when domain
                           starting and shutdown)
* src/qemu/qemu_driver.c (Update 'sharedDisks' when attaching
                          or detaching disk).