use another defined type for physical volumes

A volume group can contain multiple physical volumes; however, passing
an array of strings into $pv results in an error, since defined()
requires a string argument.

For example, this code fails under Puppet 2.7.19 with puppetlabs-lvm 0.1.1:

    $lvm        = {
       lv_var' => {
        ensure => 'present',
        vg     => 'vg_root',
        pv     => ['/dev/sda2','/dev/sdb],
        fstype => 'ext4',
        size   => '100G',
      },
    }
    
    create_resources('lvm::volume', $lvm)

but works after applying this patch.