virt: split CPU spec parsing code out into helper method

The get_vcpu_pin_set method in nova.virt.hardware will
parse the value of the CONF.vcpu_pin_set parameter. The
code for parsing CPU specs is needed in a number of other
contexts, so split it into a separate parse_cpu_spec
method that accepts a string, instead of hardcoding the
CONF.vcpu_pin_set string.

Blueprint: virt-driver-numa-placement
Blueprint: virt-driver-cpu-pinning
Change-Id: Ia9df3a7bebd691efc2143e141b2c6eef2f1c326c
Merge "virt: split CPU spec parsing code out into helper method"