qemu_block: Validate number of hosts for iSCSI disk device

qemu_block: Validate number of hosts for iSCSI disk device

An iSCSI device with zero hosts will result in a segmentation fault. This patch
adds a check for the number of hosts, which must be one in the case of iSCSI.

Minimal reproducing XML:

<domain type='qemu'>
    <name>MyGuest</name>
    <uuid>4dea22b3-1d52-d8f3-2516-782e98ab3fa0</uuid>
    <os>
        <type arch='x86_64'>hvm</type>
    </os>
    <memory>4096</memory>
    <devices>
        <disk type='network'>
            <source name='dummy' protocol='iscsi'/>
            <target dev='vda'/>
        </disk>
    </devices>
</domain>

Signed-off-by: Rayhan Faizel <rayhan.faizel@gmail.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>