qvm-create.1: improve --root-move-from description
qvm-create.1: improve --root-move-from description

Describe by its difference from --root-copy-from, and be clear about
what the difference is.

Signed-off-by: Yann Dirson <69228ceb6f6706d8bbf6135fa4ad3d70b54c46ed@free.fr>

marmarek/qubes-core-mgmt-client,marmarek/qubes-core-mgmt-client,marmarek/qubes-core-mgmt-client