KVM: selftests: Confirm exposing MTE_frac does not break migration

When MTE is supported but MTE_ASYMM is not (ID_AA64PFR1_EL1.MTE == 2)
ID_AA64PFR1_EL1.MTE_frac == 0xF indicates MTE_ASYNC is unsupported
and MTE_frac == 0 indicates it is supported.

As MTE_frac was previously unconditionally read as 0 from the guest
and user-space, check that using SET_ONE_REG to set it to 0 succeeds
but does not change MTE_frac from unsupported (0xF) to supported (0).
This is required as values originating from KVM from user-space must
be accepted to avoid breaking migration.

Also, to allow this MTE field to be tested, enable KVM_ARM_CAP_MTE
for the set_id_regs test. No effect on existing tests is expected.

Signed-off-by: Ben Horgan <ben.horgan@arm.com>
Link: https://lore.kernel.org/r/20250512114112.359087-4-ben.horgan@arm.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

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