[minfs][blobstore] Allow mkfs to reformat FVM partitions

Previously, running 'mkfs' on an already-allocated
filesystem partition would result in failure, since
"ioctl_block_fvm_extend" would fail on a previously
allocated slice.

This patch adds a mechanism for filesystems to reset
all slices (except the first) prior to reformatting.

Change-Id: Idad89b3e91a05cde169e036b216e648c92bcba3e