virsh: forbid negative vcpu argument to vcpupin

virsh: forbid negative vcpu argument to vcpupin

The vcpupin command allowed specifying a negative number for the --vcpu
argument. This would the overflow when the underlying virDomainPinVcpu
API was called.

 $ virsh vcpupin r7 -1 0
 error: numerical overflow: input too large: 4294967295

Switch the vCPU variable to a unsigned int and parse it using the
corresponding function.

Also improve the vcpupin test to cover all the defects.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1101059

Signed-off-by: Jincheng Miao <jmiao@redhat.com>
Signed-off-by: Peter Krempa <pkrempa@redhat.com>