parallels: implement virDomainDefineXML operation for existing domains

parallels: implement virDomainDefineXML operation for existing domains

Add parallelsDomainDefineXML function, it works only for existing
domains for the present.

It's too hard to convert libvirt's XML domain configuration into
Parallel's one, so I've decided to compare virDomainDef structures:
current domain definition and the one created from XML, given to
the function. And change only different parameters.

Currently only name, description, number of cpus, memory amount
and video memory can be changed.

Video device and console added, because libvirt supposes that
VM must always have one video device, if there are some
graphics and one console.

Signed-off-by: Dmitry Guryanov <dguryanov@parallels.com>