qemuDomainBlockCopy: Check @granularity to be a power of two

qemuDomainBlockCopy: Check @granularity to be a power of two

https://bugzilla.redhat.com/show_bug.cgi?id=1206479

As described in virDomainBlockCopy() parameters description, the
VIR_DOMAIN_BLOCK_COPY_GRANULARITY parameter may require the value to
have some specific attributes (e.g. be a power of two or fall within a
certain range). And in qemu, a power of two is required. However, our
code does not check that and let qemu operation fail. Moreover, the
virsh man page is not as exact as it could be in this respect.

Signed-off-by: Luyao Huang <lhuang@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>