Remove kvm as a hypervisor_type argument

QEMU is used whether specifying either qemu or kvm as an argument to
hypervisor_type. Therefore, remove kvm as a supported argument.

Change-Id: I89f2b907fa868e50c2b65c2bf4a8e414242dd9fc
Closes-bug: #1195361
Merge "Remove kvm as a hypervisor_type argument"