target/riscv: Respect MPRV and SPRV for floating point ops

mark_fs_dirty() is the only place in translate.c that uses the
virt_enabled bool. Let's respect the contents of MSTATUS.MPRV and
HSTATUS.SPRV when setting the bool as this is used for performing
floating point operations when V=0.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>