block rebase: initial qemu implementation

block rebase: initial qemu implementation

This is a trivial implementation, which works with the current
released qemu 1.0 with backports of preliminary block pull but
no partial rebase.  Future patches will update the monitor handling
to support an optional parameter for partial rebase; but as qemu
1.1 is unreleased, it can be in later patches, designed to be
backported on top of the supported API.

* src/qemu/qemu_driver.c (qemuDomainBlockJobImpl): Add parameter,
and adjust callers.  Drop redundant check.
(qemuDomainBlockPull): Move guts...
(qemuDomainBlockRebase): ...to new function.