qemu_shim: Require absolute path for root directory

qemu_shim: Require absolute path for root directory

The virConnectOpen(), well virConnectOpenInternal() reports an
error if embed root is not an absolute path. This is a fair
requirement, but our qemu_shim doesn't check this requirement and
passes the path to mkdir(), only to fail later on, leaving the
empty directory behind:

  $ ls -d asd
  ls: cannot access 'asd': No such file or directory

  $ virt-qemu-run -r asd whatever.xml
  virt-qemu-run: cannot open qemu:///embed?root=asd: unsupported configuration: root path must be absolute

  $ ls -d asd
  asd

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Kristina Hanicova <khanicov@redhat.com>