baremetal: VirtualPowerDriver uses mac addresses in bm_interfaces

VPD uses bm_interfaces.address instead of bm_nodes.prov_mac_address
to lookup a target VM. This patch follows the change in
I5ce940e7127aa3b29ba7802612938dc5dbc3152f, which moves
bm_nodes.prov_mac_address to bm_interfaces.address.

Change-Id: I2bab869b282958ee9fd0e4776928e56351018324
baremetal: Change input for sfdisk

To prevent sfdisk stopping with "bad input", use a empty string
instead of "-" (and doing so requires using "," as a separator).

Fixes bug 1166112

Change-Id: I5e4cffaa326fbafd1583d55744f921a40b3ae76f