The number of vCPUs for a guest must be between 1 and the
maximum value configured in the domain XML. This commit
introduces checks to make sure that passing count <= 0
results in an error.
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1248277
Signed-off-by: Luyao Huang <lhuang@redhat.com>