qemu: Fix two issues in qemuDomainSetVcpus error handling

qemu: Fix two issues in qemuDomainSetVcpus error handling

Issue #1 - A call to virBitmapNew did not check if the allocation
failed which could lead to a NULL dereference

Issue #2 - When deleting the pin entries from the config file, the
code loops from the number of elements down to the "new" vcpu count;
however, the pin id values are numbered 0..n-1 not 1..n, so the "first"
pin attempt would never work. Luckily the check was for whether the
incoming 'n' (vcpu id) matched the entry in the array from 0..arraysize
rather than a dereference of the 'n' entry