cli: Escape comma in values

When a command line option uses a value with several items we use the
comman character as the delimiter of name value pairs, for example:

  # update vm myvm --cpu-cpu_tune-vcpu_pin "vcpu_pin.vcpu=0,vcpu_pin.cpu_set=0"

But some values may actually contain commas, for example the "cpu_set"
value may contain a list of CPU indexes separated by commas:

  # update vm myvm --cpu-cpu_tune-vcpu_pin "vcpu_pin.vcpu=0,vcpu_pin.cpu_set=0,1"

The CLI can't currently support this, as it assumes that "1" is a name,
instead of taking it as part of the value.

To allow this kind of values this patch introduces the backslash as a
way to escape the delimiter (the comma), so that the command can be
written as follows:

  # update vm myvm --cpu-cpu_tune-vcpu_pin "vcpu_pin.vcpu=0,vcpu_pin.cpu_set=0\,1"

Change-Id: I4bcc5944e3f4236b3495acc02801acae7ec82eb2
Bug-Url: https://bugzilla.redhat.com/1088914
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>