config: add support for paravirtprovider

[oliverkuntze]

this adds support for paravirtprovider property to set the
paravirtualization provider in virtualbox.
default value for paravirtprovider property is set to "default".

implements $vb_paravirtprovider variable in Vagrantfile and
config.rb.sample.

background:
with virtualbox on windows and vm_cpu > 1 boot process hangs during vagrant up at "SSH auth method: private key".
starting the vm directly from within oracle's virtualbox admin tool shows that booting hangs at "x86: Booting SMP configuration:".
reason is that coreos-vagrant doesn't set the vm's paravirtprovider property.
because of this the vm is created in virtualbox with paravirtualization (under "system->acceleration) set to "legacy".
this iin turn causes the vm not to boot.

[oliverkuntze]

sorry for my mistake.
without wanting it protobus' patch #316 got into my pull request.
this is my first time using git. i should have forked the repo, created a feature branch and than created a pull request for that branch right?
since i didn't want to wait for protobus' pull req to be accepted i merged it into my fork. apparently that's why it is now also in my pull request.
should i do anything about it?

[jbaez001]

Ran into this very same issue -- tried the changes from the commit (paravirtprovider) and it worked as intended. 👍

[frizz925]

Any updates on this PR? I'm experiencing the same problem on Ubuntu 18.04.1, it would be nice to have the ability to change the paravirtprovider.

[bluermind]

Thanks @oliverkuntze. This got me fixed when having two or more cpus in initial config. It would be nice if this PR could be merged into the main branch.