storage/lvm: fix reporting lvm command error
storage/lvm: fix reporting lvm command error

Escape '%' in error message, as required by Admin API.

Fixes QubesOS/qubes-issues#3809

marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin